I think the thing that made it notable in a series that is known for being pretty stark and not very uplifting is that ultimately Bill and Frank won. The audience already knows that Joel is a person who has no hope for anyone, let alone himself. He's a survivor who's walled himself off from nearly everyone around him. He doesn't even think twice about justifying killing Ellie when he points out to Tess that she's gonna have a shit future anyway and he's probably doing her a merciful deed.

Bill did what Joel has failed twice at. But Bill showed that it can be done, and people can still win. One of the final scenes where Joel asks to see Ellie's bite again is him looking one more time before he lets himself make a leap of faith. And none of that would have been possible without seeing that Bill and Frank not only carved out something functional, but something that they clearly both loved.