There's a whole subsection of app devs that will just stop making apps for android. Getting graphene or a chinese phone with android won't mean anything because all you will be running is old version of apps since there will be very few devs interested in developing for the platform. The vast majority of people owning an android will be buying "certified" devices which means they can no longer run these apps, essentially drying the whole ecosystem up, which I think is google's goal.
Hopefully this means that a third player will join and provide a truly open android platform.
If an OS is owned and controlled by a single company, it's never truly open. No matter how much they claim it is.
edit: I should clarify, I'm talking about devs that develop for third party stores exclusively. Usually privacy conscious or devs whose apps aren't allowed on the play store for this and that reason like tachiyomi.