I reckon there is a strong fanbase of people in their mid twenties to late thirties who were kids or teenagers when the series was originally aired.
I think stuff you watch and "get in to" in your formative years make a big impression, in a way that TV and films you watch as an adult just don't - I watched Withnail and I, Twin Peaks and The Goonies as a kid / teen, and have such a deep affection for those shows and films, maybe cos they made such an impression cos of my age, and they continue to remind me of my past. I was an adult when i watched Blue Velvet, MD and other Lynch works and they will NEVER capture my imagination in the same way, purely cos I was an adult when i came by them.
I think us Peaks fans will stay loyal til we are old and grey, and maybe the series won't have such an active following in 60 years when we are gone, but I don't think it will be forgotten, I think there will always be some off the wall kid or cool auntie who introduces their mates or family to TP.
I also think it will last because it is deliberately styled in a retro, quirky way, so it won't date, because it already looks deliberately dated. When you look at how dated a series like Lost will look in 50 years, because all the styling is contemporary, Twin Peaks won't suffer from that, as it looked very kitsch and '50's from the word go.
The Norwegians are leaving!