I think they'd treat him relatively well in the WWE. Bruce Prichard loves him. I just don't know how his character would transition to WWE TV. He wouldn't have the same freedom in promos, which is the whole reason why he's as over/popular as he is.
I don't feel like he would have quite the same passion that makes his promos so believable if he's neutered with a scripted promo.
I think he would get treated well initially if Cody has pull by the time MJF potentially gets there. Things may change with AEW talents' perceptions of WWE if Cody's aura starts to fall off in WWE. He doesn't exactly have the size that Vince likes to push at 5-11. I mean AJ Styles is 5-11, but MJF is no AJ Styles. And AJ Styles isn't far removed from being in one of the dumbest tag teams with that worthless paperweight Omos.
I also think it's more as likely he gets treated like most every other WWE talent, he may have a run where he gets booked and treated like a favored son, but most of the time he's going to treated like a second or third-rate talent jobbing out to whatever the franchise talent of the day is or being booked to challenge or win meaningless titles like IC and 24/7.