It would break pretty much every Mac game on Steam, Origin, and the Epic Store. Sure it could give you access to what good games exist on iOS but devs would have to optimize for the change in form factor and I don't know how many would be willing to do that. Plenty don't even bother for the iPad despite it being much less work.
I doubt most AAA or indie devs see enough of a market on MacOS to see recompiling their games for ARM be worth it.
I am an Indie Dev so I am definitely only seeing this from my perspective. Games on Steam would be lost (or at least heavily disrupted) But that isn't so bad because those weren't driving much revenue to begin with. If Macs switched to ARM the main benefit would be that lower end Macs would all ship with substantially better graphics performance than they have now. It would create a new open space to push games into which would help with visibility and it would create a much better performance baseline. The transition would be a little rough, but I think the final effect would be worth it. As far as recompiling goes, the middleware that most Indies use these days makes that a pretty low-cost affair. It's actually the larger studios that might have a larger problem with porting their stuff.
Instead of the Mac being a small odd attachment to the PC market, it would be a more naturally integrated attachment to the mobile market. I think that provides a more natural way to create a pool of users who would be interested in playing games on Macs in the first place. Apple Arcade's cross-platform nature seems to be an attempt to start bridging that gap and creating a market of people who consider their Macs as a thing they would play games on. For me personally, right now that is something I never think of doing on my Mac. I game on my PC, my consoles and my mobile devices, but I don't even look at my Mac as something to play games on.
Of course all of this depends on Apple actually treating games with a level of respect that they have never really done constantly.
