I think if payload was played with the same ruleset for payload that overwatch does it'd be servicable. but still far far worse than 5cp or KOTH. 6s was designed for highly mobile fights around multiple map objectives including controlling flanks and grabbing the various points, its more dynamic.
Payload could be pretty fun in prolander though, having to play around a cart would allow non-6s classes to be more powerful than normal and would open up an entirely different experience. It wouldn't be 6s though, and would probably just devolve into who's got the better sniper.