I am not at all convinced Apple wants to go that route so the future of the mac doesn't look scary to me it looks pretty great.
They have to tread carefully because anti-trust, and they'll make all the relevant noises, but I think it's clear that the end-goal is only allowing app store apps to be installed.
But tbh the same is true of Windows. They're further away, because history, but they've already played with this once and will do so again.
Instead, they'll push iPad to be usable as a laptop alternative for more people.
Apple has no reason to turn the Mac into a total walled garden. They already have that in the iPad. Walling off macOS would mean getting rid of the Terminal and all of the other developer tools. For what? They would be cutting off their nose to spite their face.
Gatekeeper, requiring Developer ID, deprecating kexts, T2 chip, deprecation of the inclusion of Tk/python/PHP. We are slowly boiling frogs here. It's being squeezed into the direction of iOS.