It isn't a monopoly though. Even ignoring the Blizzards, Epics and GOGs of the web, any developer can host their game on their own Web site and market it completely independently of Steam and keep 100% of their takings.
The monopoly on storefront argument holds water in mobile land where side-loading a game is not possible/easy. In the world of computers though, I don't think the same standard applies.
I suspect that the majority of voters never wanted to leave in the first place. Results-wise, there was like 1.2% in it. And the leave voters were more likely to actually turn up. The problem is that too many "remainers" didn't actually vote.