Doesnt this create an arms race situation where every 'critical' app claims a larger diskspace than necessary, just in case, and accelerates the issue?

Kinda sorta, but there's a limit where users will typically install X apps and apps of Y size need Z extra space to update. User content would fill up the rest. I would imagine a typical 256 gb phone is probably over this limit and people who take lots of videos/photos just need to clean up their phone a little more often.