Point but still - there are feats I wouldn't mind always requiring GM approval for swappage nonetheless.
Actually no, the prereq is a feat. So it IS accessible to everyone, it's just that the Planar Philosopher or Gate Warden backgrounds get a head start. If those truly don't match your concept, then you have to wait a bit longer to be an Agent of Order, but you can still be one.
Not to mention - given that the Elemental Scion feats are freely swappable into any of the featless backgrounds, I don't think it would be a big stretch to let you do the same with Outer Planes Scion too, if you really don't want PP/GW.
It's not equally good for everyone - even if you can only apply it once per turn, more chances to hit means more opportunities to do so. Better still, once you land the restrain on them then all the rest of your attacks get advantage, so saying multiple attacks aren't rewarded or even that they're rewarded less doesn't really gel for me.