I've never been able to make myself use Reddit, so maybe this is an obvious point, but that sounds like a fairly apocalyptic "whole point" for the exercise of an attention span. Which, I realize, is the goal, but is probably not great for the people thus enmeshed, no?
I doubt it has anything to do with attention span. I have a very short attention span, and the only reason I tend to prefer consistent interfaces is because many websites still don't support dark mode. I'd say the attention span is actually the reason I do not use the reddit redesign, because it just wastes my time at every corner and I hate UIs doing that. If your website doesn't let me consistently middle-click links it's going on my mental blocklist.
My guess is that people prefer having everything on one site because it makes the whole UX consistent. If we're being honest apps, websites etc. are wildly different to navigate and use, and when you're used to one site's behaviour and quirks it makes visiting other sites a bit more cumbersome. Especially if you're not using adblockers.
