The point is that no one forces you to jailbreak your iPhone. If you like the walled garden - great! Keep using it! But I've been in plenty of discussions where iPhone users are literally against jailbreaks at all and in any form.
Windows malware that says stuff like click Yes on the warning bar or box shows how well that works.
I want a device I can give the elderly in my family that they cannot be easily induced into doing insecure things on. If there were an unlockable iPhone and a locked iPhone I want to give them the latter. It’s not enough for me to tell them not to scroll to General and click seven times on About to activate developer mode. I would prefer to just give them a device that can’t do that.
My own parents are still relatively young and my in-laws are sceptical enough of other people that it’s currently safe but it won’t be so forever.