Preamp tubes pretty much last forever. Power amp tubes not at all. If your power amp tubes are dying and you don't want to spend excessive prices for new tubes, consider putting the tube amp aside and getting a good solid state power amp until tube prices come back down.
I very much favor the vintage 200 wpc Sumo Andromeda amps, which typically run about $900 on the used market.
Tubes becoming hard to get?
An article in today’s WSJ talks about tubes for both guitar amps and stereo gear becoming scarce due to Far East and Ukraine/Russian tensions saying most tubes originate from China, Slovakia, & Russia. As such, people are beginning to Hoard and retailers are beginning to limit purchases.