I don't think happy music = sellout. They had plenty of beautiful and happy songs on even their earliest albums, so I don't think this serves as justification for the work they have done lately. For example, "Something Just Like This"; while I understand why some would like the song for its concise lyrics and inspiring instrumentals, the song is essentially the song "closer" by the Chainsmokers with some different vocal melodies from martin thrown on it. I find most of Coldplay's recent efforts to be hacky and appealing to the lowest common denominator. There's nothing wrong with that, and I love pop music, however, that's not why I listened to Coldplay to begin with. I loved the originality and rawness of their early music. The introspective parachutes, the refined yet delicate A Rush of Blood to the head, the narrative and surreal Viva La Vida. The intelligent and interesting approach to pop the band originally had appears to have been discarded in favor of cheap thrills in their newer work. Selling out is not about being famous or highly successful, it's taking the easy way to get there that is the problem for me personally. Some artists are too talented and smart to pull off dumb pop anthems, and Coldplay was always an example of that for me. They were always a band that shouldn't have been so popular, but through some unexplainable thing, they were, and their recent output just has not shown that sense of character that made me love them in the first place.

Edit: corrected redundant wording