The railway scene is constantly changing, and this is one thing that attracts some enthusiasts to it. In my life (I'm just short of 17), I've seen the Crown Point 150s transferred away and replaced with 156s, the 317s supplemented with 379s (I now find the West Anglia Main Line a lot less interesting as a result, particularly at weekends!), the Stansted - Birmingham trains becoming 100% 170s rather than a mix of those and 158s. And then there's TOCs, I remember when my three locals were WAGN, CT and Anglia, rather than FCC, XC and GA!
The changing scene means there's always something new to photograph or something to have a few last rides on. My main interest is 2nd-generation units, such as 317s or 150s, and for me the railways will never be the same without the sounds of DC motors or Sprinter engines, but by the time all those classes have been withdrawn something new that I like may have popped up. I enjoyed riding on the 1967 and A Stock on the Underground during their last few months of operation, and am doing the same with C Stock currently. I also enjoyed riding a 379 and a 2009 Stock for the first time (as much as I dislike them!) because it was something new.
So overall I doubt I'll ever lose interest in the railways, simply because there's always something different to try out, or have a look at.