I've told this story here before, but "we don't support that" doesn't go nearly as far as you think. When they released the intel macs and boot camp, boot camp came with explicit "we don't support this or problems you have with it". At the time I worked for Apple retail and one of my most memorable customers came in steaming mad. She wanted to return her brand new macbook because it was having all sorts of problems. The sort of problems that shouldn't even be possible on a mac, and the sort of problems that were addressed by a firmware update 6 months earlier. I managed to get her to agree to demonstrate what was going on for me before we did the return and as soon as she booted the computer, it booted right into Windows. It turns out she'd bought her new mac and given it to her "computer guy" nephew to help get it set up. Nephew apparently thought that what she really needed was not a mac, but an expensive windows computer instead. So he installed boot camp, shrunk the mac partition to it's smallest size and then configured the system to boot into Windows by default. As a result, she had a macbook that had been without any recent firmware updates to solve some issues (because firmware updates run through the OS update software, and not in windows) and hadn't even installed an anti-virus software (because "macs don't need it") and so had a number of malware infections going too. From Apple's written perspective, we would have been absolutely within our rights to say "well you've installed boot camp, we don't support that and you need to use macOS instead" and sent the customer on her way and denied the return... and that would have been a shitty customer experience. So we spent some time getting the customer's computer re-configured, moving some of their important info to the mac partition, showing them the new OS, explaining what the heck happened in the first place (which involved explaining to someone who barely knew how to use email what bootcamp was and why a mac running windows isn't really "a mac" in the colloquial sense) and convincing them to give it a try for a while before deciding they wanted to do the return. Now that experience in the end turned that customer into a very good and reliable customer, but it also meant that "we don't support that" was a meaningless boundary. It also meant that the issue wasn't that the customer did this to themselves. They didn't. Someone who understands the implications of developer mode isn't what Apple is concerned with. It's the people who don't understand that, who will never see the "here be dragons" warning because they didn't enter developer mode, their "computer guy" nephew did.
Also, people already gripe about the fact that to "fully unlock" your mac these days, you need to disable SIP, and how that disables some things (but at least not your warranty). Do you really think the same people would be happy with "developer mode but no more warranty" on iPhones? Especially if someone like Epic or Facebook or Google insist that their apps need this unlocked mode and so the only way to use them is to void your warranty?