I'd think any well made tube preamp will last much longer than 20 years, though you may have to replace resistors, caps, or tubes along the way.......same with a SS preamp.
BAT, ARC, CJ, Aric Audio, Don Sachs, Supratek, Backert, et al are built with extremely high quality parts; I'd suggest that too will lead to longevity. Cheap parts won't last, better parts will.
I also like the idea of point to point wiring (Aric Audio, Don Sachs, Supratek, etc). Part goes bad, unsolder it and resolder the replacement.