City don't rely on their fullbacks to provide attacking width because, unlike pretty much every other top side in Europe, they don't usually play with inverted wingers. Instead, the fullbacks play a hybrid midfield role, generally tucking in, blocking off counterattacks and assisting in the buildup (which is why Pep often plays actual midfielders in these positions), while the wingers position themselves wide.
Perhaps we'll play with a similar system next season, in which case AWB's lack of attacking ability won't be an issue (although his passing ability would be). Or perhaps we'll continue with a diamond formation, in which case the attacking prowess of the fullbacks will be absolutely vital. Either way, there's no particular reason for us all to have to deny the absolutely obvious.
Poch plays with attacking FBs and they play higher up the pitch, Ben Davies is below AWB's in cross completion. Same with Alonso who is very attacking FB, if anything he is a wingback with very good delivery and he is below AWB in cross completion.
Also Pep use fullback to provide width but they don't usually play many crosses. They rely on cut backs and FBs are used to stretch the game.
There is more to the attacking game than just cross completion and key passes, if so we won't be working very hard to replace Ashley Young who is good in these stats. Both Crystal Palace FBs have played around 50 crosses all season, which looks like a tactic than weakness. Maybe this has something to do with not playing Benteke as regular striker anymore and playing Zaha and others as attackers.
Going by your stats, Roberton is 25th in cross completion per 90 mins, Young is 6th. Young is 7th and Robertson is 8th in key passes. Anyone really believe Young is as good as Roberton as attacking Fullback?
Obviously AWB is not as good as TAA, Robertson in crosses (well he is not better than them in most attacking aspects) but doesn't mean he isn't good in attacking game. He is good in carrying the ball, making driving runs taking on defenders, has good link up play. For now his final pass is not good but that's something that can be improved with more options in the final third and experience.