My take is the shift to mobile is what really caused the biggest change in the Internet.

By the mid/late 2000s, the UX for surfing the Web was mostly pretty good. Navigating to another site was as easy as typing in its URL or issuing a quick search.

Navigating the Web on mobile is much more difficult -- even 15+ years later simple things like browser tabs are a nuisance on mobile. Typing URLs is still a pain. Etc. That extra friction for more traditional Web browsing led to users preferring simpler apps that they didn't have to navigate from, including content that was less interactive. And social media companies were quite happy to exploit that and create endless feeds of content. People who create content followed suit and went were the eyeballs were.

Also, before mobile, using the Internet largely meant sitting down at a computer and browsing the Internet somewhat interrupted for a period of time. With mobile, Internet use became more far more disjointed. You might browse for a series of 2 minute spurts in between other activities, rather than having a dedicated sitdown browsing session. This also rewarded the 'feed' that instantly provided you something to do, without you having to put in effort to find it.

> Navigating the Web on mobile is much more difficult

I tried a foldable phone thinking it would solve my issues with mobile interactivity, and learned that it's not the small screen that hinders me. It's the terrible mobile input methods in general, particularly surrounding text manipulation and entry. I didn't keep the phone since it didn't solve my seemingly unsolvable problem after all.

Not just difficulty on the browsing side, difficulty on the presentation side, too. It took a while for anybody to figure out a decent presentation for forums on mobile devices, and in the meantime the experience was terrible. Frankly, you can get away with much worse UI on desktop, which made hacking together a webpage much easier.