There seems to be trails that are terribly popular, then not mentioned much, Off the top of my head –

Carn ban mor
Auchindrain
Ciaran path (still mentioned occasionally, but was mentally hyped up a few years back)

Can trails go out of fashion?.

Trails can be over hyped, after making a special effort to find something it a bit dull then I guess they go out of fashion.
Trails change too become easier or less doable, weather impacts people choices and to some extent fuel costs make people think about staying closer to home etc.