I don't see why they would do this. If they just want to sell iPads, they could do that tomorrow. Just release XCode for Linux and Windows so that devs can create iOS apps and call it a day.
Obviously they still intend for the Mac to remain a general purpose computer or they wouldn't be putting this much effort into it.
When GiorgioG speaks of a closed, iOS-style ecosystem, I don't think they mean literally running iOS on laptops.
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.
People have been fear mongering this for a long time now without anything to show for it. People see the writing on the wall but honestly I don't really buy it.
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.
Exactly. They still need something to develop iOS and MacOS themselves on, so unless they want to move all their internal lower-level development over to Windows or Linux (which, IMO, doesn't seem like something Apple would do), they'll need to continue producing something resembling a general purpose computer.
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.
Obviously they still intend for the Mac to remain a general purpose computer or they wouldn't be putting this much effort into it.