Lower AC is necessary. In turn-based rpg misses aren't funny for most people. It's frustrating when you miss every attack for several turns in a row, but if you lowered AC you have to increase HP or fight will be to easy. You can't just add new enemies because no one wants to wait 10 minutes for next turn. Changes in rules are necessary if Larian wants to sell a game for people unfamiliar with d&d. You can't make game for few thousand people if you create AAA.
What exactly are you smoking? Few thousand people? Dungeons & Dragons is the single most valuable fantasy RPG IP in the world.
And those fans are reasonably looking for D&D 5e in this game, not D:OS3.
Do you really think at least 1/4 of the D&D fans will buy this game? Most of players will be casual. Larian has to sell far more copies than last time if they don't want to end up like Obsidian.