Agreed.
Of course Branthwaite is young and probably nowhere near as good as he'll eventually be. But a good season in central defence in an Everton side that's had to defend a lot to stay in the league proves he's a decent player who can handle pressure. It's not like he's competing for a place with loads of experienced centre-backs - that position is pretty much Stones the veteran alongside a selection of players with a handful of caps. Makes more sense in that context to give talented 21-year-old Branthwaite the opportunity over solid but uninspiring 32-year-old Dunk.
For me, Mainoo has promise. He's tenacious and drives forward on the front foot, and those are good qualities. But he's also defensively naive, positionally poor and his league experience is a few months in a midfield that hasn't worked out how not to be overrun. I don't think he's good enough right now for the senior squad - should be learning his trade in the under 21s, I don't think he meets the threshold to be skipping that step.