Well, you are assuming that :
1 - Everyone will land hits and spells which is unlikely
Yes, because that is what you do when calculating average output comparisons between creatures and party members. It's not 100% realistic; it's comparatively sound, which is why we do it.
More precisely: you presume that all attack rolls hit, all saves are failed, all save-out-by-round effects are saved out of by the second attempt, and all AoE damage spells, such as sphere, lines etc., hit two additional targets. That's the math that it is conventional to use when doing statistical comparisons. Is this news to you? This has been the standard way of doing things for many years.
2 - Ignoring legendary reactgions
No I specifically mentioned legndaries where they were relevant - as part of the enemy's potential damage output calculations.
3 - Ignoring that a group VS one enemy is a rare occurrence. A young dragon will probably have Kobold servants which will rush to help his master.
Yes, because you were talking about individual characters and claiming that 5e suffered individual hp bloat, causing combats to last into dozens of rounds. It doesn't, and that's incorrect. It's disingenuous for you to make claims like that when they are clearly and demonstrably counter to actual evidence; what is your motive for doing so?
I don't care about your older edition specific figure comparisons; I care about you being factual and honest about the edition that you're denigrating and slandering; you currently are not being so. For example, if we talked about porting your legendary 2e figure to 5e, we'd talk about doing so in the exact same way that we're talking about everything else here... we don't maximise his hp, we state his expected average hp at max level... which in a 5e translation is 150Hp, if we take him to the level 20 cap, or 225 if we give him an extra 10 hit dice to emulate level 30, or 300 at level 40 - which, I must point out, is not a thing in the bounded statistics of 5e as of yet. Leveling past 20 does not give you ANY more hit points at all in 5e; you gain other perks, but you don't gain further hit dice. In 5e, a theoretical 'level 40' wizard has 20 hit dice, and with 19 con averages 150hp, the same as a level 20 wizard...
Regardless of which you choose, it doesn't sound as though you are actually very familiar with 5e at all; most of your information is incorrect (there are many domains of dread in 5e, for example. We only have published adventures for Barovia, as the most centrally famous, but the lore that's been published in other official 5e books supports and even explicitly calls out the existence of many others). It sounds much more like you've developed a negative opinion of it from afar and are repeating the fallacies you've adopted without any substantiation.
So, the question I'd ask would be, if you are set and determined to down-talk 5e, and are not prepared to acknowledge where your information is false, or to revise your position on any part of the conversation as a result of those falsehoods... then what is your actual purpose here? Why have you decided to do this? What's your motive, and what do you get out of it, and how can we help that in this discussion? I'm genuinely curious.
Edit: As part of this I suppose I'm motioning that we're a little bit off topic from the origin of the thread, which I think was the complaint that we can't realistically know or test anything about how the high level magic will work in BG3 while we're level capped... with the underlying understanding that high level magic has a major impact on the game, and we really should have a means of testing and providing feedback on it before official release; this, at least, is a sentiment that I don't entirely disagree with.
