I think it's several different things
- a visual design aesthetic
- specific gameplay mechanics
- "legacy" systems and software
I think each of them can differ in whether they're fixed or not. Generally I think that in game design, retro is fairly anchored when it comes to visual aesthetics and gameplay design. "Boomer shooter" mechanics and visuals, pixel art games, etc. I suspect we'll still see those 'retro' games in 20 years, and probably not see e.g. Ubisoft-style open world control-point-capture games being called retro.
Consoles though, I do think shift into retro status very consistently. I think there are people who would even consider DS or certainly GBA games as retro already.