> There was nothing preventing them from ‘locking down’ an intel based macOS

On Intel Macs, you can install Linux and Windows if you are unhappy with macOS. So they couldn't really lock it down. With their own custom ARM processors, they can prevent the installation of other OSes. Thus, like with iPhone or iPad, you will be stuck with the OS they provide. With the M1, so far you can only run other OSes on VM.