Besides, the main reason Valve is investing so heavily in Linux and Proton is so their destiny isn't tied to someone else's platform. MacOS is just another someone else's platform like Windows is, with the same threat of getting rug-pulled by a first-party app store that spooked Gabe Newell[1] into investing in Linux in the first place.
The magic of Proton from a consumer point of view is that it just works for basically every game, sans those with Kernel-level anticheat stuff. This means thousands of old games that haven't been updated in years will work.any games that don't have active developers.
So Apples solution works for new games but isn't a practical option for compatibility for existing games.
Personally 99% of my Steam Deck usage is with it docked. I do mostly use a controller, but also have it hooked to the same USB switch as my PC so I can hit a button to move my keyboard and mouse over.
Baldur's Gate 3 is the first game I ever ran on my Deck that did not run very well, though. Most stuff I've played runs at 60fps at my external monitor's 1920x1200 resolution. That in addition to not liking the gameplay on BG3 much made me not continue with the game, though I may revisit it someday.
I think you picked as an example one of the games that actually has a native Mac version?
Or is it a well hidden wine package? I've played it start to finish on Macs only and it looked too smooth to be emulation to me.