Sure, browsers had three decades of adversarial testing to evolve into sandboxes, but what are you going to do in case of something like the xz backdoor in a desktop application? It's no longer a hypothetical in 2020s.
You're going to be hacked. There's no useful middle ground between letting programs modify how your computer works and not letting programs modify how your computer works.
Do you want to see the dancing bunnies? [Yes] [No]
Apple apologists keep making the excuse that Apple has to provide no side loading because if there was any single way to do it, all scammers would be making all grandmas do that. They're correct.