It would make macOS more of a general-purpose OS, would increase the amount of functionality from which third parties would benefit, but Apple themselves would likely not. That would increase the number and variety of tech support requests, ever so slightly but still, and would introduce a few new attack surfaces.
Instead, Apple's strategy is to tighten the macOS more and more, and turn it into a specialist OS completely controlled by Apple, with a few companies like Adobe and Ableton licensing access to its internals.
I've been using OSX since 2003, and developing on it for more than ten years. At no point have I seen anything that it's reasonable to call "tightening macOS", let alone the absurd claim of complete control except for an inner circle of elite companies.
The closest thing would be adding the attestation system, so that unsigned binaries have to be explicitly given permission to run... once. That's a security feature which trades a bit of convenience for a lot of protection, especially for the average user. I have no problem with that sort of thing.
I see this sort of sentiment very frequently from non-users of the operating system, but never from those of us who actually use it. Go figure.
Apple used to be a lot more developer-friendly company. It is part of what got them where they are now - the fact that so many developers use Macs, which in turn encourages business software vendors to support Macs
Stuff like this is of little interest to ordinary users (at least not directly), but appeals to developers
By de-emphasising the developer is experience, they are undermining one of the factors that got them to where they are today
It would make macOS more of a general-purpose OS, would increase the amount of functionality from which third parties would benefit, but Apple themselves would likely not. That would increase the number and variety of tech support requests, ever so slightly but still, and would introduce a few new attack surfaces.
Instead, Apple's strategy is to tighten the macOS more and more, and turn it into a specialist OS completely controlled by Apple, with a few companies like Adobe and Ableton licensing access to its internals.