I’ve been an active member of various communities and forums over the years (excluding reddit), and I never actively left one. It was always just a realization I hadn’t been to the site (or really aging myself, BBS) in a year. Most of them where daily habits at one point.
That's how communities get big without becoming cesspools or going over the top on the moderation. See recent examples of Discord and Twitch, which started out targeted to gamers, and have gone mainstream from there.
The tl;dw is that if your "alternative" platform doesn't launch with unique and valuable features of its own to attract users away from the original platform then you will only attract toxic people who get kicked off the original platform as the first users of your "alternative" platform (because they are the only ones really in need of an "alternative"). This creates a self-reinforcing pattern where non-toxic users are repulsed from your alternative platform by the toxic users so growth only comes from more toxic users. You can clearly see this in the dynamic between Reddit and Voat.
I'd remind you too that a lot of people - including myself - don't see this "toxic" dynamic the way you apparently do. Our stomachs might turn at the sight of much of what is on reddit nowadays just as yours might when you see the front page of Voat.
Maybe userbases are generally dividing as we seek providers with which we are more politically aligned, with decreasing focus on the bells and whistles they have on offer. That's their value.
Side note - I thought Victoria Taylor (/u/chooter) left for that competitor, but it appears that she was at WeWork and now at LinkedIn (as of August - just in time!)
Victoria never worked at Imzy though (and didn't leave for anything - she was fired). She was at Cake (https://cake.co/) for a while, I think that was between WeWork and LinkedIn.
