how long should you use tubes in an amp?

As tubes age and they lose power, is the degradation linear? My Mesa Baron uses 6 tubes per channel, and the speakers are efficient so I never use more than 10% of the output power, even on peaks, even in all-triode mode - and most of the time, it's far less. The Baron has meters, so it's easy to see this (in fact, it has switches to reduce the meter range by a factor of ten - they would never budge, otherwise). So if no more than a fraction of the full output power of the tubes is required, why not use them well past their "normal" lifespan - and just turn up the volume? Do the tubes lose some dynamic or other capability, or is it just a "smooth" decline in power?