I agree it'll be hard to replace him and I agree he's been fantastic this season with plenty of plaudits, infact I really like him and think he is an elite shot stopper and that is a primary role of a goal keeper.
However Ederson and Alisson have better clean sheet percentages than Hendo whilst being better in other areas such as sweeping and distribution but these are the 2 keepers from the 2 best teams in the league right now. Alisson has played 20 games to Hendo's 27 which is why alot of pundits are favoring Hendo in their TOTS.
In my opinion Hendo is somewhere between the 4th-7th best keeper in the league, 1/2/3 being Alisson, Ederson and Lloris. I'm not even counting De Gea here even though he is fantastic but doesn't deserve a top 3 spot at the moment. On form Hendo is better than DDG but it's hard to discredit a keeper who has won 3x Man Utd player of the season awards (a testament to how awful they've been post-Fergie).
Keepers who are on the same level for me are Pope, Schmeichel, Guaita, Leno and Dubravka. However out of all these Hendo is primed to push to become a top GK in a top team, for me it's only a matter of time, I only feel he is over rated because people act like he's miles ahead of our team but our defence this season has been unreal and deserve just as many plaudits as Hendo.