There's something people seem to be forgetting about Luna's guilt, it wasn't just the bad things she actually did as Nightmare Moon. It's also what she attempted to do and what she could no longer do because she got herself banished.
She tried to murder the mane six multiple times, as well as trying to kill her sister and cause the apocalypse, if someone tried to kill you and failed you wouldn't treat that like nothing just because they didn't succeed.
We also see Equestria a thousand years after she was banished, but back then Luna was one of the only two alicorns and an element bearer. She had very important responsibilities as a ruler of Equestrian. Even if Luna didn't think ponies appreciated her, that doesn't mean she didn't have important royal duties that would cause chaos if they stopped being done. Who knows how ponies reacted to one of their immortal rulers suddenly disappeared. We know for a fact there's at least one thing that Luna can do that Celestia can't, which is protect ponies dreams. It wasn't like Luna just threw a temper tantrum and then got over it, she forced Celestia to suddenly adapt to ruling equestria alone. I'd say that Celestia couldn't use the elements anymore after the trauma of banishing her own sister, but that would be veering into serious speculation territory.
Loved the episode, by the way. Though the end would have had more impact if it were a bit less rushed.