Rooney never had a shot at the Ballon d'Or because his competition was Messi and Ronaldo and because he was severely injured in arguably his best season where he'd be a major contender had it not happened, so this point is moot.
Mbappe is younger and better than Rashford, but this doesn't tell the whole story, a footballer's development is very unpredictable and so often young phenomenons stale while other players have a later peak.
Right now Rashford is up there with the most promising players in the world, so a Ballon d'Or is not out of question, although I wouldn't put my money on it happening.
Truth be told, it shall be a very underwhelming battle for the title of best player in the world. The possible contenders are not even close to Messi and Ronaldo. They are not close to recent legends like Xavi, Iniesta and Robben. They are not close to the legends of the 90s or 00s either.
Today we have good players with very high tactical functionality, but I miss the sheer talent of the last decades.