It's a good question. I'm not sure there's a single answer, but the big thing is probably corporatisation and amalgamation in the internet. In the early days of the internet, it was absolutely viable to just buy some tiny site, post your own webcomics, mention it in a couple IRC chats or online forums, and watch a forum and community flourish - it was actually achievable for the average user, and I know because I watched average people start up and get fans and have a nice time. Most communities tended to be no more than 100 regulars in a web forum. The first decade or two of the internet really was a time where any schmuck off the street could make something fun and it would enjoyed and participated in by others. They didn't make much money if any, but that was never the point.

As SEO, Corporations, Google, Reddit, Twitter, Facebook, Instagram so forth started to monetise and dominate, its now basically impossible. Money-making is the primary objective and so monetised things have been pushed on us and these big corporatised communities are the only way to hear about anything. Nowadays you've very, very little hope of making it big anywhere on the internet without aggressively advertising and monetising it. And if you're one of the 0.01% that still become popular while steadfastly refusing to do those things, then the massive tide that is the internet will probably break or shit on it anyway.

As a fun fact, I know two people who moved across the world and got married because they met on some stupid tiny pixel art forum two decades ago. I don't see those kind of personal connections being made online anymore.

No surprise that a leftist blames corporatisation for 'ruining' the internet, but as someone who's lived just long enough to watch that transformation happen, it seems the biggest contributor to losing the good aspects we once had.