I've mentioned before why sentimentality is dangerous. I would say it's more insidious, because people don't ever want to analyze it, nor are they educated on why its harmful. It's one of those things that sits on the periphery of understanding so our eyes glaze over it but I assure you its behind a lot of really - really - ugly behavior.
It is not easy to define "What makes Baldur's Gate, Baldur's Gate". I am sure it is possible, but I don't think fans of the series should be required to write a doctoral paper analysing titles they like, in order to be allowed to not like what the "sequel" is doing. Because here is the insidious thing:
Baldur's Gate3 is called Baldur's Gate3 because someone wants to benefit of the good will and "prestige" of the IP. Turning around and dismissing fans of the IP because the sequel doesn't live up to expectation set up by BG1&2 IS a crappy thing to do. If you make Lord of the Rings related project, people will pay attention because it is Lord of the Rings related project. You can't then turn around and complain that people compare you to Lord of the Rings.
Is it a worthwhile thing to complain at this point? To me BG3 not being a Baldur's Gate was clear from the moment it was announced that it is being made by Larian, and for everyone else it should be clear since first gameplay videos were shared. One might dislike direction the series is being taken, but complaining at this point about it seems like wasting time. StarWars is dead. Star Trek is dead. Baldur's Gate is dead. It won't stop fans from complaining, though.
And of course, what is and what isn't a worthwhile sequel will vary from person to person. Personally, I think the very appeal of the games is too different. I liked Bioware games. I am somewhat liking Baldur's Gate3 - but for very different reasons. Not every WW2 shooter is the same thing. Not every gangster film is the same thing. Thief reboot on top of being a bad game, was a bad Thief game, because it wasn't an immersive sim - there are similarities that can be made, but overall, the Thief reboot doesn't even attempt to recreate the appeal of the IP it was using.
BG3 might be an RPG, and might be based on DnD, but that doesn't necessarly make for a good Baldur's Gate game. The best explenantion I can give, without having to analyze the details is: Infinity Engine games had three series - Baldur's Gate1&2, Icewind Dale1&2 and Planescape Torment. They run on the same engine, most of them were based on same ruleset, I am pretty sure they shared assets. They are not the same game series - they have different focuses and different appeals, even though there are a lot of similarities. Some people like all of them, some people like some series more then others, some only like one or two. Modern example: Disco Elysium, Pillars of Eternity, Pathfinder: Kingmaker, Divinity: Original Sin2 are top down RPGs but are fundamentally different games - in appeal, in design, in tone, in priorities, in gameplay they allow for. And while I adored Disco Elysium and Pillars of Eternity, if PoE3 will come into existance, and will play like Disco Elysium - I won't be very happy about it, even if game, as such, is pretty good.
Another example: Let's use Thief example again. I really like Dishonored 1&2. It is immersive sim, which I always like, and it is clearly inspired by Thief games, which are some of my fav games of all time. It would, however, be a lousy Thief sequel, if it were sold as such. You use IP, you create expectations, often unreasonable ones, if it is a legendary series.
BG3 might be made for Larian fans. Might be made for DnD 5e fans (though that's doubful considering how many changes there are) or people who want DnD-like coop experience in digital form. I am confident in saying that it is not made for Baldur's Gate fans. I am confident in claiming that one can adore Baldur's Gate1&2 and all it's legacy, and not care one bit for Baldur's Gate3. There will be overlap and people who like both original Baldur's Gates and Larian's take on it (I for one liked all Infinity Engine series, I liked Bioware games, I liked Obsidian games - in general I consider my taste to be fairly wide, and there is a chance that I might like BG3 quite a bit, if it keeps improving) but that doesn't mean that people disliking BG3 are dillusional, or unreasonable. Unrealistic, perhaps, if they think they can change fundamentals of BG3 at this point.