I never got why Frutiger Aero got so popular as a ‘nostalgic’ aesthetic, when it’s basically the Windows Vista, GNOME 3 (the awful rewrite of GNOME 2), KDE 4.0 (the buggy, emo black rewrite of KDE 3) look?

It was the lowest point of computer graphics. Who the hell is nostalgic for that? Probably just kids that had their formative years in those ~2-3 years. Not sure you can even call it a niche.

I’m a fan on the vaporwave/Windows 2000/XP aesthetic, the Vista era is when everything started going to shit.

Well yes, nostalgia is based on age. The people who are nostalgic for that grew up with those sorts of UI.