I never said they declined, not sure where you got that from. I'm saying we had 3 goalkeepers who were among the best in the world until their decline (or they left). De Gea hasn't been the best in the world the last 2 years, nowhere close, thats why talk started coming up for the goalkeeper position again. Schmeichel left while on top, Van der Sar retired while still being a great goalkeeper. I never presumed anything, was young during Schmeichels time at United so its fuzzier but more than old enough at Van der Sars to have watched every single match he played for us!
Anyway, my point is this. We had 3 pretty much all time greats as our goalkeepers over the past 30 years. The others in between lasted not very long as the starters. So unless Henderson can reach a Schmeichel/VDS/DDG level, I'm not sure if he'll be the starter more than a few seasons, assuming he even takes over from De Gea. Which is far from a guarantee. De Gea isn't that old, slight chance he wants to move back to Spain possibly so Henderson gets the gig, but if not, DDG is the better goalkeeper and Henderson won't wait around for him to leave forever.