First, Discord caused problems a thousand years prior but not FOR a thousand years. Reformation with him happened because Celestia saw a use in him and wanted him to be reformed. It took over a season for him to actually stick to it and that was from seeing he wanted friends.
Diamond Tiara was being raised a certain way and when the veil pulled back, she did not actually enjoy it. She redeemed herself because she actually wanted it and just needed someone to show her the way.
Starlight’s reasoning was not done out of malice. She believed herself a savior in the opener. In the season finisher, she honestly believed she was only ruining Twilight’s friendship. She did not wish to harm Equestria at any point. She was just short sighted.
Tempest Shadow was hurt and abandoned. She tried to sacrifice herself in the end when she saw where her loyalties to the Storm King landed her.
Cozy Glow has zero remorse. She doesn’t understand friendship because she didn’t WANT to understand it. She only saw power. Everyone else understood it. She had no remorse. She had no regret except losing. She isn’t redeemed because it’s clear she doesn’t want it. She had friends and she only saw them as a stepping stone.
See, I don’t think there’s anything inherently wrong with reforming different villains, but I think it all comes down to the execution.
I had no problem with Discord being reformed because he WAS punished first for his actions (being turned to stone) and even after that it wasn’t an instant 180 (aside from Fluttershy, the Mane 6 took a WHILE to warm-up to him and even Fluttershy really had to work hard at it).
With someone like Starlight’s redemption, what always bugged me about it was not only how weak her backstory and motivation was considering all everything she did in Season 5, but her whole redemption was just a tacked-on “Hey, just be our friend, we’ll instantly forgive you, no questions asked!” in the last five minutes of that finale with no notable consequences. I didn’t really mind that she got redeemed, but the WAY she was redeemed was just incredibly weak.
I get that friendship is the central driving force of the series, but if you resolve every conflict with little more than a simple offer “Be my friend,” than fatigue just sets in to that kind of stuff.
@genervt
Reminds me of the chubby kid in Deadpool 2 who ended up making friends with the biggest and baddest CGI character in mutant prison, just so they could wreck shit together.