Preamp mods/Why capacitors?

...not only capacitors: fuses(especially on the output stage), internal wiring, resistors...
Capacitors will loose their dielectric abilities within the time and definitely need to be replaced.
For the best performance you shouldn't wait the end of their life and use even less than half(the same I would recommend for tubes).

Capacitors are very much like batteries in how they function and even in chemistry (i.e. electrolytic) so no wonder they wear out.
Some caps dry up over time especially if they get hot (i.e.Tantalum).

There as been a lot of improvement in capacitor technology in the past years,older designs did not benefit from those.

The funny thing is that your PC or cel. phone probably uses better components in some cases than your high end audio gear it's KHz vs. GHz , the good thing is those technologies eventually trickle down to Audio gear but rarely the opposite.

Dare to object Miket2us since to manufacture ...GHz capacitor is much simplier than ...KHz. Even two opposite leads in PC board can be used as ...GHz capacitor and participate in producing a carying high freequency. Moreover these are two completely different issues.