I thought about this comment, and realized that somehow, I just don't care so much anymore. Instead of worrying about what I left behind, I'm looking forward to what's ahead of us.
I think it's because even before the whole 3d-party-app drama, there already was this undefined feeling that Reddit's best days are behind it. Maybe it's the effect of ad money and monetization, or it's the inevitable trend towards low quality content that comes with mass adoption, probably it's both.
Whatever the cause, in most subreddits, the old Facebook-style rot had already set in. Once-cool subs now being an endless barrage of tired memes, bots farming karma, and people being assholes. The things I joined for years ago, the engaging discussion, random encounters with amazing experts, the cutting-edge internet anarchy, it's all already long gone.
When I opened the app (Baconreader in my case), I only did it out of habit, to then spendy time scrolling through an endless list of things that made me slightly go "heh".
So, maybe most people will stay on Reddit for now, and probably I will have to leave behind certain communities instead of finding direct replacements. But I see that as a good thing. As long as even just 2% of Reddit's users make it here, I'm excited it will grow into something much better than what I left behind.