I don't think it's a matter of ui convention. Doing things the Whatsapp way does not work for such large amounts of chat channels. They're just doing it because people are used to it..

I think there's 2 kinds of usecases for matrix. One tries to just replace Whatsapp etc as a person to person chat network. It aims mainly at novice users. The other usecase tries to bridge as many chats into it as possible to have a central place of interaction. This is obviously for more advanced users. Still, the element team seems to aim for this usecase too with their Element One service.

The ui of element and the other high profile clients works great for the former but poorly for the latter.

I use gomuks by the way but I would like to see images (with a click if needed). Gomuks actually shows them but very low res.

I could be wrong because I never used them myself, but isn't that Spaces feature supposed to address this at least in part?

I tried it when it first came to Elements but I didn't manage to set it up so that stuff from each bridge went into its own space. It seems to be more something for matrix communities on the wider network, not for local bridges. Perhaps it's improved now but it didn't work for me when I last tried it.

I should give it another try though. I'd kind of forgotten about it because after the first try I've hidden the entire vertical bar for it, to reclaim some screen space. So I haven't given it any thought lately.