That's how all these vendors justify their actions once they decide it's time to kill off their competition and impose arbitrary limitations. But how real is this "damage", actually? How big do you think the overlap between 'users who want to install custom operating systems', 'users who have no idea what they're doing' and 'users who would loudly complain after an obvious mistake on their own part' is? The #1 source of reputational damage for vendors is people breaking something within their controlled environments. The bar to run a custom OS is already so high for average people that it will never matter or be a social engineering attack vector, just because of how niche it is. We've been able to run anything on PCs for over 40 years, custom firmware for phones goes back at least 25 years, and despite all of this, the world hasn't gone up in flames yet. The security pressure exists, but it's minimal compared to everything else. The real reasons are profit-driven.
>The bar to run a custom OS is already so high for average people that it will never matter or be a social engineering attack vector
>We've been able to run anything on PCs for over 40 years
And the PC platform has had terrible security. Do not use it as an example of everything being fine. You are forgetting the malware for PCs which would spread by writing a custom operating system on a removal media. Then that removable media is plugged into another PC and when that PC boots up it boots that operating system which goes ahead and infects the OS installed to the hard disk along with any other removable media.