My theory (which is definitely not originally mine) is that mobile devices have driven a huge shift in usage patterns towards low-effort content consumption. Smartphones are everybody's go-to when they're temporarily bored, so social media on the phone is less of a deliberate destination and more of an idle snack to alive someone's boredom. Even when people do engage, because they're often doing so on-the-go in short bursts and don't have access to a real keyboard, the engagement is typically very low effort. TikTok is a perfect testament to this shift IMO.
I'm thinking that desktop-only requirements would lead to less engagement over all, but higher quality engagement when it does occur. If there were a ranking system or some other kind of algorithmic content serving, it would be less skewed towards content that you can fully engage with in two seconds. And the userbase would skew towards more intentional enthusiasts who deliberately seek out the site, rather than someone looking for a quick distraction while they're bored for 15 seconds.
Hacker news does probably the best job at this without actively trying to prevent phone usage, but it doesn't exactly do much in the way of fostering a community like old-school forums.