The MacBook is a general purpose computing platform. The iPhone and iPad are not. Locking down the Mac will make it unusable for many, many people. It will indeed be the death of the platform, as most devs abandon it entirely.
As a Mac user who develops high performance scientific applications portable between all UNIXen (Linux/Mac/BSD), I still can write my code pretty easily on the platform.
I personally don't use Homebrew, et al. but, I have a Linux VM which handles that stuff pretty well.
I didn't develop a Mac specific "application" though.
My experience has been the opposite. Of all the people I've worked with using Macs, all of them were developing cross-platform open source software. I've yet to meet a single developer making MacOS applications.
Neither do game consoles or the large population using Windows based systems.
I never cared for brew on the occasional moments I get to use Apple computers, XCode and default tooling is more than enough.
Which is like what the large majority of developers targeting Apple devices actually care about.