Two points here: One can wonder why WotC went with Larian given the intention of not adapting the 5e rules, or why they did not enforce a faithful adaptation, which would be better for their product (the D&D books). IMO that's a bad strategy from Larian and WotC. D&D has an estimated 40 million playerbase, if they went with a faithful adaptation, they would be drawing from the D&D and CRPG public, and a fraction of this would make a smash hit. DOS playerbase is much smaller, so it seems strange to focus on those, given that probably a large share would buy the game regardless.
This analysis has a couple of assumptions:
1) A "faithful adaptation" is required to draw the D&D crowd
2) The perception of BG3 is that it's more Larian/DOS than D&D
We have to remember the bias intrinsic to the people posting on these forums right now - we are literally a minority of die-hard nerds posting endlessly on a forum for a game that's in EA, months in without an update
There are many D&D players who don't even recall or understand the abilities of their own characters, nevermind care about what the RAW ruling of X and Y is. What "Larianisms" that sticks out to some people here like a sore thumb, can just be a "oh look, a fun house rule" for many other players.
There are lot of D&D players who care so much more about the narrative and roleplaying more than the mechanics. In fact, on tabletop, a lot of discussion here would be labelled as people trying to "rule lawyering" a DM. Look at what Larian HAS been focusing on - the characters, their romances, and the narrative design. It would seem like for the majority of the public, that's what they care about most, over a few mechanical implementation.
I think it's safer to assume that both Larian and WoTC, who actually has the numbers (market research and the in-game feedback/data), has run them, and calculated this is the best strategy to move forward with. We don't actually know DOS 2 sales (steamspy says 2-5 million), and the 40 million D&D playerbase number is an overaching number with no details. If WoTC has done their market research correctly they'd have a good understanding of what the breakdown of that 40 million is, and have segmented it properly to know who are likely to buy a D&D videogame, and thus strategized accordingly to their needs.