But is that really what PINE64 should be trying to do? So far their support hasn't come from the "mass market". It's come from a niche market of open source hackers trying to build and support various Linux distros for mobile devices. Why does improving mass market appeal have to mean alienating your existing supporters?
Not as long as they ship Manjaro as the default OS for their hardware. Rolling-release distributions are not fit for mass-market use.
Also: I've seen some hidden costs of supporting custom OS installs being discussed, i.e. procuring extra chips to allow open boot. This may have factored into Pine64's decision.
That's a false dichotomy, nobody is demanding that users be forced through "custom Linux install" (whatever that means). The problem is also not primarily that Pine64 have chosen a "flagship" distro, but how they and said distro behave towards the other options. I'm sure the quality of the flagship distro is massively improved by making life hard for the project that did useless things like making the camera in the phone work...