There are not so many RPG enthusiasts, judging by the sale of other games of this type.
Agreed. And the indie-game market has served that niche very well - within that niche market, those indie studios command just as much, if not more, brand power as Larian.
Just look at the excitement and praise heaped at Wrath of the Righteous (CRPG enthusiast) and Solasta (5e enthusiast) - why would Larian go compete where niche, established competitors are? The best way for a company like Larian to succeed to to build on the momentum it created with DOS 1 and 2 and continue to scale larger.
Let's also be honest, speaking as a CRPG enthusiast myself, we're probably one of the most notoriously vocal, hard-to-please, snobby type of gamers lol who ultimately won't pay any more for the game than random gamer #868.