Slugs to be you then, I guess. :P
In all seriousness, the graph shows different species as fraction of total uses recorded. Since the paper is mostly about mice, and behavioural differences under different circumstances, it being unfair to the slugs is probably not such a big deal here.
Honestly, yes.
More information available at once, more fine user control, less chance of fucking up. If you’re going to be spending big, at least make sure what you’re doing is the thing you wanted. Desktop sites are still superior in that regard. Until the mobile-first trend turns everything into an enshittified mess, that’s the way I’ll keep doing things.