As a hyprland user, why should I switch to Niri? Is the appeal based on the "inifinite windows" feature

I don't think its for everyone, the paper metaphor either works for you or it doesn't.

That said, the other big benefit for me is it breaks a lot less often than hyprland and its ecosystem seems to (and I don't just mean bugs here, I also mean things like config file format changes). And this isn't a slam on hyprland - I was only ever mildly annoyed by its breakage.

"written on rust" is one pro, the other is the native scrolling thing, I appreciate workspaces with a bit of leeway. If you're already on hyprland it'll be an easy switch