I agree. Big centralized platforms (youtube, Reddit, the chans, HN to some exent) mean that the internet riff raff (i.e. people with no interest in the niche and who may feel like trolling) are free to show up in any given niche area of the platform (e.g. a subreddit or someone's channel) and then shit all over it. A community that would have been perfectly peaceful on a newsgroup, IRC channel or forum now has to contend with literally anyone from anywhere else on the platform. The old 4chan quip about /b/ leaking is now the default state on basically every platform because the platforms are so big even a small trickle of trolls is a full on flood of crap. People are getting fed up. I think there will be some decentralization in our future.
The real problem with practically every internet community is its moderation, because the worst people with the least business doing that job inevitably float to the top and are then impossible to dislodge without voting with your feet and creating an entire new community.
The corollary to this that remains unspoken about the OP's thesis is: because of the mass reach of the major platforms, the "toxic" effects now have escaped being an internet-only phenomenon and now are manifesting in meatspace.
It's overly reductionist to attribute Trump and Brexit just to online trolls, but it's folly to ignore their effect at the margins (because both Trump and Brexit were relatively close calls, influencing a 5% middle ground definitely mattered).
On traditional Usenet there is a kind of unspoken rule about never taking online disputes offline. Everyone was supposed to understand that it's just a game.
A good portion of the ragamuffins who are overflowing the online world have not such concept. Everything is personal.
I don't think it's that at all. People have always been able to visit different websites. It still happens today. The rise of mega-platforms is concerning, but not for the reason you cite. It's always been possible for "riff-raff" to notice a community.
