You’re glossing over a lot of complexity that the GrapheneOS team is doing for you
Not meaning to gloss over, it’s their hard work that makes it a nice experience! It’s always been the case that some devices are more moddable than others, comparing the ones that are simplest is the only comparison that really makes sense though.
As for users moving en masse it obviously wouldn’t be trivial, but the theoretical removal of side-loading wouldn’t happen overnight so there would hopefully be plenty of time for more solutions to pop up.