not that the original fetish becomes mundane, just that they need more and more to hit the same high.
for example my friend with a rubber fetish. Started off with balloons. Then progressed, glvoes, odd items of clothing.. that became full on catsuits. then hoods. then two layers. then 3. then more and more to the point of needing breathing tubes etc....
not saying its the same for everyone, but it does seem to go that way.
From my experiences, and ive known some people heavily into it all, ( im not, but i find it interesting, and they are happy to talk about it) its much like sexuality, being gay or straight, it was there from the beginning.... they cant change it and i didnt chose it, its just how they are.
i think, a general rule would be, dont do anything you dnot want to, dont feel you have to, and dont do anything that doesnt rock your boat too. Also, avoid anyone where the fetish is more important than the person they are with...