Agree with all this. Loaning him to Sheffield United for another season just makes sense on every level.Realistically if Henderson was here next year he wouldn't start the season as #1. DDG would and Henderson would have to wrest that mantle from him.
If he does, great. If he doesn't though then what? He's now isn't getting the game time he needs to develop and has hindered his chances of becoming England's #1. Does he go out on loan again? Stay on the bench for a few years until DDG leaves? It wouldn't be a case of "well if he can't beat De Gea then he isn't good enough anyway" either. He might be good enough, he just wasn't ready yet. And for the sake of rushing him in off the back of one PL season at a lower level you've hindered the development of a promising player.
Whereas if we send Henderson out on loan again his career can continue to flourish, his chances of securing the England #1 spot increase, we end up with double the amount of PL games to judge him on and we get another year to assess DDG's current standing. If Henderson has another successful season (and becomes England #1) while DDG has a third season in a row of underperformance then there's a different dynamic between the two, with Henderson in a better position to come in and supplant DDG. But crucially if DDG has a good season, Henderson's development hasn't been hampered and he's still potentially in a position to take over from DDG down the line.
Whatever about whether Henderson would wait three years on loan, I haven't seen anyone suggest he's unwilling to wait another year. So leave him be.
From their side they get to keep arguably their best player for another year and look to cement their place as a Premier League side (even perhaps a top half/mid table side). They'll also be able to postpone buying a first choice keeper another year and can use their funds elsewhere - so they'll definitely be up for another season loan.
From his side he gets another full season playing every week in the Premier League (possibly also in Europe we shouldn't forget if they qualify) in the lead up to the Euro's where he must now be confident of becoming England's no.1 in the near future.
From our side he continues to develop and should hopefully be an even better keeper by summer 2021 (still with 2 years left on his contract) whereby we can evaluate the situation between him and Dave with more basis to make that judgement on (e.g. whether Dave returns to alien-mode, whether Henderson continues to improve etc).
I really don't understand any argument to the contrary unless you are of the opinion that De Gea is finished and Henderson is ready right now (which I'm not sure on either of those). The suggestions to sell him this summer or have him as a backup next season make no sense to me.