[Prospective Vision] Improving the approachability of data-race safety

[dima_kozhinov (Dmitry Kozhinov)]

@Andropov

But I wonder if this could become an instant performance downgrade for apps when switching to use the Main Actor by default (with dropped frames and minor but noticeable UI hitches).

I agree. Everything should be Main Actor by default unless you start using concurrency. I am strongly against executing async functions in Main Actor by default.