My theory is that at some point after Viva La Vida the band decided there wasn't any rock bands or music groups for children, so they decided to be original and become the first rock band aimed at children.

Proof of that we can find in Mylo Xyloto and then releasing a Comic Book, which lets be honest, its not aimed at adults... it might have adult themes like Disney Pixar movies like CoCo or Up, but it's mainly aimed at children with some minor mature content for the parents... That's Mylo Xyloto.

Then it seems that they got overly attached to this children Genre and they kept it very prominent in Ghost Stories, Again some mature themes for the parents to enjoy like a Pixar Movie but the overall work is mainly for children... Example "Magic", The "Ink" music videos, "A Sky Full Of Stars".

Then it was now obvious when they released A HEAD FULL OF DREAMS literally with photos of themselves when they were kids in the artwork.

I guess they want people to literally grow up with their music hand-by-hand by just saying "Goodbye" to older fans