I think you're oversimplifying the issue.

First, we don't know how long it'll take Harry to revive Hermione. Given the fact that he's made a couple of completely groundbreaking, paradigm-shifting discoveries within a year of being introduced to magic, we have no possible way of predicting how long he'll take to achieve this particular breakthrough. Our own knowledge of magic, of Harry's potential, and of what resources might become available to him in the future, is insufficient to model him in this much depth.

Second, people mature at very different rates depending on their circumstances. Typically, more challenging circumstances make for greater maturity, as long as the person doesn't break down altogether. Being revived after getting murdered and a time-skip has the potential to make Hermione mature a great deal very fast (though she probably won't enjoy it).

Third, Harry's own maturing process so far seems rather non-standard, with rapid growth in some areas and a striking lack of it in others. Again, it is very hard to model what he will become as time goes by, especially given the number of significant character-shaping events that keep getting thrown at him.

Fourth, there is such a thing as friendship across maturity levels. It won't be the same as it was before, but really, we already knew that. Too much was happening to these two from the start for their friendship to maintain any one static form, both in terms of having to respond to external events and in terms of having to learn to deal with each other's somewhat alien ways of seeing the world.

Fifth, what joke is on Harry? He's Harry James Potter-Evans-Verres and his best friend is dead. He's doing it because it's the right thing to do and because she, in particular, matters that much to him, not because he expects things to go back to the way they were.

Or possibly he'll have done enough nasty things "for the greater good" that by the time he gets around to resurrecting her he'll have become someone she can't respect.

Well, Hermione is (well, was) slightly older than Harry, and she seemed to have entered the romantic stage already. A couple years to let Harry catch up might not be such a bad thing.