Amps go wrong - valve and solid-state both. A good valve amp is more durable in the long run than a solid-state model and much of the servicing can be done by the user. While the valves themselves wear out and can occasionally be problematic, the amps are designed to be rugged and durable, when things wear out or blow, you replace them and they keep on going. Testament to this is the fact that most of the amps built in the '50s and '60s are still going strong, and, with proper care, will probably last
at least the same again.
Modern SS amps are mostly throwaway technology - something goes wrong, they unclip the main board and replace 1X new - which usually costs about the same as replacing the whole amp with the newer models. Chances are, if something goes wrong with a 5 year old solid-state modelling amp, even if you find someone willing and able to work on it at component level, it will be "old tech" - and any specialised ICs will be unavailable.
According to Alan's tips, some valves should be replaced after two years
That's a bit of a "safe" generalisation on my part, I'm afraid. There are a lot of factors at play. Lifespan is affected by how hard the valves are pushed, whether they are preamp valves or power amp valves, how hard the amp is pushed and the type of amp circuit.
Preamp valves handle less voltage and go through less abuse and will last much longer than than the power valves The only exception is the first preamp valve in a high gain amp - it gets hit pretty hard, so you want to make sure that a good quality preamp valves goes in there. The nice thing about preamp valves is you can swap them out yourself, no need to take it into a tech to revalve.
Power valves are bigger, handle more voltage, are located at the back of the amp, get very warm, and tend to get abused a lot more than the preamps, so they have a shorter lifespan in general. Overall, the biggest factor in valve life is how much you use them and how gentle you are with how you use them. If you crank your valve amp every day for hours on end then your valves will fail much faster than if you only played for an hour or two a week. Used hard, power valve in some amps could last under a year, while just ticking over in other amps, you could get five years of use out of them.
N.O.S. (New Old Stock - Old, but never used) valves do last longer than the current production valves - A good NOS rectifier valve should last 20+ years and IMO is critical for good tone and operation, a preamp valve probably 5 - 10. NOS power valves are harder to find and seriously expensive, so I usually go new production for those. Given how long they usually last, NOS valves usually are worth the extra money. In most cases, they will sound better too.
In your situation, I'd look at a self-biasing or fixed-bias amp, which you can change any of the valves in without needing a tech. Vox are good fixed bias amps - a tube goes funny, you just whip out the faulty one and replace and you're up and running again.
