Again and again, there's only one way to prove whether Henderson can do it - let him play as a United's no. 1. If you think his stats at Sheffield are irrelevant, extending his loan is meaningless. He'd keep putting in great numbers or probably win a PL with them, and you could still argue that wouldn't translate into a success at United.Stats playing for Sheffield United don't translate into him being a success at Manchester Unitef.
Ben Foster had really successful loans and Fergie said he'd be United and England no. 1 - look how that turned out!
As for listing people like Neuer and Alisson - don't be silly. I clear was saying is Henderson the best we could get, there's a world of other keepers out there beside the obviously unavailable ones you disingenuous named.
I don't understand why Foster is kept being mentioned. He had never enjoyed success and proven himself as much as Henderson in his loan spell. We also had a reliable van der Sar back then. The situation is barely comparable.