When I worked at my local bike shop we promoted Seven Cycles as a custom alternative to the retail stuff we were pushing. I know we had several riders who had their bikes well over 15+ years. The titanium frameset lasted easily that long, along with some wheel sets, brake upgrades and component upgrades along the way - but overall the frame and fork seemed to really have some longevity.

They aren't cheap but the investment I've been told is worth it:

https://www.sevencycles.com/