Maybe. Certainly people within Apple would have thought about Linux for this. But Apple would need to provide some form of mechanism for unlocked bootloaders anyway to facilitate kernel/driver developers and security researchers, so I'm pretty sure other OSes is not the main reason they do this.
It does work out for Apple in the end. Their current standard 10 years' support will look quite short now Moore's law is dead and their hardware has barely any moving parts. But they'll shush some complaints if up to date third party OSes are available in 2030.
