yes. There are service centres and 3rd party car mechanics who will absolutely be able to replace the entire engine for you or parts of it (I have done so on a couple occasions).
so your point is exactly counter to the one you're trying to make. With the iMac Pro, not only is the manufacturer refusing to do out of warranty paid repairs,
But they're also providing reasoning why he can't get a 3rd party to repair it even if he wanted by limiting who gets the parts and who can sell them.
No, you've obviously misunderstood what I said.
If you walk into the dealer you bought your car from, and ask "hey, I want to buy a complete engine" - they'll probably tell you to go away.
Instead, you'll need to go via a mechanic, or their service dept, to do so.