[blooalien]

I dunno about pure Arch, but when I was using Manjaro (an Arch derivative) upgrading often was never a problem for me, as the system stayed ridiculously stable (in a never crashing or breaking on me in any unexpected ways sorta sense), so I imagine as long as Valve makes it a point to test each upgrade they push for any unexpected oddities before they push it to the end-user, there shouldn't be any problems there.

As for the games themselves, Valve's been workin' hard on their Steam Linux Runtime containerized game thingy, and Proton sorta "containerizes" (WINE prefixes) things, so that allows for a reasonably stable environment for any given game which already runs well in those scenarios.

TL;DR: I doubt Valve will have too many problems keepin' this thing fairly stable from an end-user point of view unless you start hackin' around on it changing too much, then it's all on you what happens next. ;)