In my experience, installing alternate OS's on Mac hardware has never been frictionless or satisfying anyway.
Obviously they still intend for the Mac to remain a general purpose computer or they wouldn't be putting this much effort into it.
Rather, they foresee OS X becoming a system where you can't run programs that haven't received Apple's blessing and been brought through Apple's store. Blessings that will be denied to software like youtube-dl.
As to why they would do this? A combination of the good of most users, who will enjoy protection from malware and viruses; and the irresistible temptation of a 30% cut of all sales.
They added System Integrity Protection (SIP) in El Capitan in the name of security which clamped down heavily on what people could do with their system, limiting hacks that allowed for modification of system applications, etc. Outwardly you can claim that this is a sign of them making it so people don't have control over their system. The reality is that it can be turned off by anyone that cares enough to do so.
They added checks to inhibit installation of unverified executables from the web. If you don't have a signature from Apple, you can't run it. That surely means they're taking control, right? Well, except that it doesn't actually stop you from installing the software.
They keep adding checks to inhibit users from endangering their system (such as much more granular permissions in Mojave/Catalina), but they have not made it impossible to execute any arbitrary code you want on their system if you want to.
Tablet dominated world never materialized.
Post PC era isn't here, the PC is dominant still. Even iPad Pro got laptop like, than any laptop got iPad like. iPad's sales are either stalling or declining.
Yes - Apple clearly wants to keep that laptop market and be general enough to be useful. But general purpose is for general public, not your average HN reader.
It's not only about POSIX. After the X years of planned lifetime (with proper software/OS updates), will there be any solution to extend the lifetime (which is what I used Linux for, on > 10 years-old laptops) ? I guess there will be no solution against planned obsolescence...
And with regards to control and privacy : will Apple finally give-up their policy of deciding (and tracking) "for your own good" which apps you are allowed to install and launch ?
It would be if they take some effort to support it.
Oh, definitely. But the Linux (or alternate OS) fans are not really on Apple's radar. OTOH, they do a good job of keeping some core binaries up-to-date, like zsh and Vim, and they did appeal about getting good compile times during the M1 release event, so they consider POSIX users part of their target market.
Last I checked, I can compile and deploy an iOS app without the need for anything POSIX.