Pillars of Eternity 1 and Pathfinder: Kingmaker were both in the same order of magnitude as DOS2 saleswise. Its not clear to me how you define mainstream if NOT by sales?
According to SteamDB, DoS2 has sold about 5 times more copies than Kingmaker, and about 4 times more than PoE1. This isn't even accounting for releases on consoles ans iOS. DoS2 has like 20 videos with 1 millions+ view on Youtube, PoE1 has about 2 and Kingmaker only has its trailer reaching this milestone.
Also, I am not familiar with PoE but PF is a system made specifically to be as exhaustive and faithful as possible, while DoS2 has a way simpler system that is more appealing to casual gamers.
I can objectively say than yes, DoS2 can be considered mainstream as opposed to comparable games in the past decade.
I'm willing to concede that it is "more mainstream" than those based on sales. I am not willing to concede that it is the "only" mainstream cRPG based on those numbers though.
If you change your position to "most popular cRPG" of the decade then I will agree with that.
