Mac App Store apps only…
There's nothing stopping Apple mandating apps go through the App Store right now. ARM has nothing to do with it [I still don't understand the question.]
I'd argue that the Mac platform is pretty much crippled right now. App-Store-only apps would be the least of our worries.
Sandboxing was the wrong answer to the wrong question.
If you heard the ATP discussion about security procedures causing everything to grind to a halt, so that you can't even get an Open or Save dialogue without the OS having to consult Apple's servers for the most-up-to-date malware list.
Mac OS is pretty well stuffed.
It's a wonky, rickety old barn, patches bolted on every which way, up and sideways - with an out-of-control Catalyst tractor crashed into the foundations - about to topple the whole thing to the ground. Remember, Apple thought those first Catalyst apps were good apps, just that the developer's choices were not to everyone's tastes.
MacOS has been in maintenance mode since iPhone arrived, just enough patches to maintain minimal interoperability with iOS - and don't get me started on Touch and Pencil support. Mac should be the -premium- Touch and Pencil experience! And iPad an adjunct for when you want to get away from your desk!
Apple likes to brag about how big its Mac business is, even bigger than it was when iPhone arrived, yet the platform is run less effectively than ever. It is artificially held back to boost failing iPad sales. Not that iPad is a superior experience, just that it's more profitable. This is a mammoth miscalculation/failure of Apple management and we're all paying the price right now. The future? I'm not optimistic.