I don't really see Larian changing the core game design prior to release. Neither should they, since those that have purchased EA could see exactly what they were buying from the early gameplay videos; it has not changed.
As with most of the more fundamental game changes that different players want, complete 5e accuracy is more likely as a post-release optional mode or from the mod community.
I wouldn't call "high ground giving advantage" a core game design. It's a mechanic. And there is a difference between "not sticking completely to the rules" and making adjustments that significantly throw off the balance of the system. Getting advantage is one of the most significant things in 5e: it typically cost a spell or ability (see the 9th!!! level spell that grants advantage for the day), and importantly it does not stack
A very simple change that would fix a lot of problems is giving height/lowground a +4/-4 bonus instead of advantage. Mechanically, this is the ~same as advantage. BUT this method wouldn't invalidate the usage of the dozens of spells, abilities, feats in 5e that grant advantage.
Similar for backstabbing: it is in no way part of Larian's "core game design" and invalidates a lot of spells/abilities that grant advantage.
I didn't engage with the specifics of the DnD player's argument ( the mathematics of the 5e system ), because I don't play TT and, therefore, really don't care. From the viewpoint of a video game player ( which is what I am ) the core design of BG3 is the environmental design, the controls and actions available, and how the game plays, regardless of how these are realised.
The game, as advertised, includes a strong vertical emphasis, gaining obvious benefit/drawbacks from difference in height, and similarly gaining obvious benefit from attacking from behind - which is possible , seemingly, because 5e is turn-based, but has no facing, ZOC or positioning rules.
Obtaining these height/backstab benefits is not cost-free, as you have to move into position, and movement is also a resource. The positioning benefits also apply equally to all actors, and the benefits can be lost/negated by opponent movement, so considering your movement becomes, relatively, more important.
This is unlike the dozens of feats/abilities/spells mentioned that presumably will continue to pertain regardless of how actors position themselves, and are therefore tactically superior to gaining positioning benefits. As the resources used are trivially restored between encounters, at the press of a button, those other mechanisms are effectively unlimited in use.
Larian ( with this and the universal bonus actions ) are clearly of a mind that when you actually get a turn, all characters should have variety things they can do, not just attempt to hit the nearest opponent, and in all likelihood miss. You may like playing 5e TT, but it has been so over-simplified that RAW would make an exceedingly dull video game by the standards of the bulk of videogamers.
Have any previous DnD video games stuck completely to the rules? I'm not aware of any. I'm not sure how many DnD TT player groups stick to the rules either.
Strawman argument. There are many other changes to the 5e rules Larian has made which don't have people complaining about them, like Speak with Animals lasting until next long rest or until losing concentration. I don't think anyone is complaining about the special weapon attacks Larian has made as well. It is clearly not the case that people are complaining merely because BG 3 is not being a precise exact duplicate of the 5e rules.
You are completely ignoring the detailed discussion about exactly why these specific rule changes affect the balance. Do you have any defense for why 56+ sources of advantage/disadvantage with a cost or drawback should be thrown out the window for free advantage with no cost from backstab and height?
It wasn't an argument, merely a question and observation concerning the differing nature of computer moderated games vs socially moderated ones.
Were Larian to be advertising a product as a multi-player 5e play-aid with assets that allowed a social group to play their sessions online, I would clearly expect full implementation of the rules and options. But BG3 is not that; it is a Larian-DNA videogame, somewhat based on 5e Rules, the Forgotten Realms and the spirit of the original BG games. They will change whatever they need, to get the game experience they want.
There are quite a few changes to 5e rules, and this is far from the only one attracting complaint. The reaction system is a big change attracting criticism, as it the apparent range limit of 18m. As far as I can see, these changes ( and the less contentious ones ) are also designed to make the videogame experience ( in the game environments Larian have designed ) less jarring and frustrating to the videogame audience.
If there truly are 56+ mechanics for producing the same non-stacking outcome, I have to wonder, firstly, why providing a 57th is even an issue - particularly as the other mechanics are presumably "at will" and recovering resources is trivial in BG3.
Secondly, I would actually step back and wonder quite why having 56+ selectable mechanics that produce an identical, non-stacking, outcome is considered a good thing. It hardly seem to me that such a situation encourages any form of tactical, nuanced or thoughtful play. By contrast, the positioning mechanics Larian have added, that achieve the same end, could be said to encourage scouting, observation and continual ,deliberate, careful movement that would otherwise be largely irrelevent.
I understand the OP's argument, it is quite straightforward; I just don't happen to agree with/care about it, because it is an artifact of the over-simplification of 5e mechanics, and Larian's chosen path seems to work well as a video game.
The height advantage/disadvantage isn't a small mechanic change people are upset about because of 5e rule puritism. As the OP has explained in great detail it is a change so detrimental to the 5e rule set it isn't D&D. It is a cheese mechanic available to all classes including spellcasters that affects all classes to the point it makes most classes completely irrelevant. Add the "free action" nonsense that allows disengage AND an attack for all classes is simply not D&D.
If Larian make the effort of including a plethora of classes and 90% of them are made redundant by making their class benefits available to all classes by either standing 6 inches higher or behind the enemy then they are wasting their time. If they add better benefits to classes to compensate then the game gets further away from D&D.
If they start from D&D core rules and make changes based on feedback rather than including DOS mechanics it would go smoother. I don't mind changing the D&D rules if they don't work in game, don't care at all. But they haven't been tried as yet soooo. Nobody expects 1:1 creation of 5e so I don't accept this argument.
I understand this is early in the EA so I am not concerned. If these mechanics remain unchanged closer to release it will be something to gripe about. I am pretty sure by this thread alone Larian are aware of these concerns.
Sure, if you are a DnD TT player, you want what you are used to. When translated to a video game, It just happens to result in a play experience that other people don't like, and is quite unlike the experience that their existing audience are used to.
From a commercial point of view, starting with what sells and moving it towards DnD until it ceases to be fun, is a more logical approach, and seems to be what they have done.
I guess you'll probably like Solasta more, but I found it slow and dull ( well made; just not interesting to play ). I have not played TT since ADnD and DnD 2e, but I understand the difficulty in finding the right balance of complexity, interest and playability. I know 3e became regarded as over-complex, but from reading the SRD and other info, I think 5e has over-compensated to the point of becoming flat and uniform. I don't really care much for any of the classes in the PHB, since the differences are largely froth and flavour; I really have no idea what I will play when EA ends; probably roll D20 ( with advantage ) to decide