The main complaint I've seen re: SBMM comes from top level players, including creators, who don't want to sweat / they want to beat players who aren't as good as them. To me this means SBMM is absolutely needed. Unranked play shouldn't mean better players can crush those who aren't as good.
This isn't even considering the fact that creators rely on looking good play-wise for their income, and SBMM makes this harder.