Life expectancy of tube components

Theo,
other parts can fail due to the tube failure such as resistors that would have a fuse act, and DC electrolytic caps that need to be checked whether the equipment worked or didn't.
small signal parts less-likely to be a culprit but power supply as with all electronics take the largest hit.
normally for the components you've mentioned, built quality is on the top level vs. others. so for the price point if you're looking for reliability, they're the right ones.

Keeping electronics in a well ventilated / cool space will add years to its life.

MTBF (mean time between failure) depends on several things. Grade of parts used. Design complexity / execution. Care of user.

Tube components do generally operate at higher voltages than transistor equipment, but in general I don't find a big difference in lifespan between the two types.

There are too many other variables involved, such as the brand involved, owner useage, ambient conditions, associated equipment, line voltage, frequency of relocation and so on.

Think of cars as an example. Some cars are better built than others, but even a model with a great reputation can be easily run into the ground in the right circumstances.

A month or two ago I finished rebuilding a 50 year old tube amp for my second system. It is working like a champ. On the other hand, I'm currently rebuilding a 35 year old transistor amp and am finding it slow work to run down the problem.

My generic advice would be to buy the component that sounds best to you that still has a good reputation.

I just retubed an old Monacor 250 last night. 7 watts of amazing power. It has been kept spotless for 40+ years and has never needed service. Cleaning the volume pots is about all it needed. Not even any hum. So my answer is this gear can last forever if you care for it. Keep in mind that this one of many vintage pieces I've played with.

There are stock amps with original caps and all, McIntosh, Marantz,and so many other amps performing great.Some brands date back to the 30's.Of course a lot depends on design,and parts quality.