Normal evolution yet somehow wasn't really a visible force on video games until streaming and is now the predominant force on pretty much every single online competitive game.

(Unless you play with cloistered private communities)

I think you're confounding streaming with the rise of solo-queue but cooperative (CS:GO, LoL...) games around 2010. Both were enabled by cheaper computing and alllowed competitive ladders to scale to millions of players. Games where you have to queue with a pre-made team (or pro teams for that matter) are considerably less toxic. Toxic streamers are more of a consequence of an already toxic community.

No, you are confounding what I'm talking about (fast discovery and dissemination of meta-optimal strategies) with toxicity. You could have 0 toxicity at all and still have a game totally ruined by the behavior I'm pointing out.

Even in cloistered private communities, "not playing the online meta" is an unstable equilibrium.

As soon as someone loses a bunch of games in a row and decides to Google "$GameName strategy", the community's unique meta disappears.