I'm agreed with all y'all. The thing for me is, I'm totally cool with Spitfire being an asshole. In fact, I actually prefer it that way.
The big problem is that the show keeps flip-flopping about it. Wonderbolts Academy was all lined up to be a monumental turning point for Rainbow Dash, where she learns that her heroes aren't what she thought they were and the cost of being a Wonderbolt involved leaving your sense of right and wrong at the door. I was practically getting up to give a standing ovation when she handed in her wing-pony pin.
Then everything goes to hell. Spitfire makes a half-hearted apology, Rainbow Dash gets a promotion and her rival, who Spitfire spent the entire episode cheering on, gets marched off in disgrace. Suddenly the Wonderbolts are wonderful again, and Dash is pleased as punch to be there. It undid all of the hard work from the first three quarters of the episode.
I think your missing the subtle adjustment to Rainbow Dash's attitude after that episode.
Rewatch any episode from season 1 where Dash talks about the Wonderbolts. In season 1 she's like "The Wonderbolts are amazing! I can only dream of being one myself one day, but I'll totally show them how awesome I am!" even at season finale, all she wants to do is hang out with the Wonderbolts and show off in order to earn her place somehow.
"Sonic Rainboom" makes her realize that actually, the dream isn't all that far off and her confidence in her flying isn't entirely bravado. In season 2 she's considerably less about Wonderbolts, and more trying to do her own thing in ponyville instead. The Wonderbolts are still her heroes, but she's stopped obsessing about them. Though she'll still get kind of excitable if you tell her a Wonderbolt will be in the general area.
By the time season 3 rolls around, shes trying to be serious about her ambitions and considers going to the academy as a major next step. For anyone who's a major admiring fan of someone or something, it's not easy to admit that your hero is maybe not perfect, or certainly not living up to the personal pedestal you've put them on! I think it's fairly accurate that Dash might struggle to get angry directly with Spitfire and co.
But Rainbow Dash struggles to excuse Lightning Dusts actions, or the fact Spitfire would even encourage it. It took putting her best friends in major danger to snap her out of trying to accept it. Even if Dash didn't completely disown the Wonderbolts, she's now quite aware that Spitfire at least, is not perfect, and stopped with the hero worship.
Notably while she is excited to train with the Cloudsdale team, she's not completely fangirling about it like she would of done in season 1. Yeah she still accepts the offer, but it's in a MUCH more casual way. While before she might of taken it without too much reservation, it really eats at her to know deep down, she's kind of leaving her friends in the lurch. Now she also knows, as a Wonderbolt she would not get the love and care she gets from her own friends. Even if Spitfire took Soarin back that easy, Dash still knows they were more than happy to ditch and run on him.
I imagine if it doesn't happen this season, in season 5 Dash may have to deal face on with what she really wants and if the wonderbolts do figure into that at all. Would be interesting to see if she ever has to face off with Spitfire for leadership.
That said I do kind of dislike that ending, it feels so abrupt. Soarin doesn't get angry or upset, he's just happy to be back on the team. I guess because the show is episodic in nature, it would be awkward to have him angry when there's no time to resolve that. But it feels a lot more jarring than Lightning Dust's dismissal. But maybe both are intentional?