All these observations I'm reading (in some other links), regarding tube "life" expectations, are IMO, overlooking two significant factors.
1) NOS vs. modern: When NOS tubes were still being manufactured, the "market" for tubes was MUCH larger than it is today (let's face it, today the entire "market" is ~95% "guitar" amps, the rest being a few die-hard tube Hi-Fi lovers). IF the current tube manufacturers made tubes that would last as long as the old ones (which they COULD), they'd be "shooting themselves in the foot" since the modern day market is so much smaller than before. It's similar to tires. They "could" make tires that last over 100 K miles, but why would they want to "kill" their market.
2) A HUGE factor that most folk are overlooking is what the tubes are subject to when amps are being transported to & from gigs, rehearsals, etc. This is ESPECIALLY a factor when combo-tube amps have had casters installed. Rolling an amp on bumpy, concrete surfaces subjects the tubes to vibrations so intense that they're getting the "sh*t beat out of them" (not to mention reverb springs as well). Compare that to a studio amp that NEVER gets moved (or very little). Case in point: I remember a customer coming into my shop one day with a nice old plexi Marshall, commenting on [until recently] how nice it had been sounding since I'd re-tubed it before. Not recognizing the customer, I asked how long it had been since the [power] tubes had been replaced. After hearing the customer say "oh, about ten years ago", I freaked at the thought of [modern] power tubes holding-up in a plexi that long (NOTE: Plexi's are known for being "hard" on power tubes). The customer then told me that the amp is always in a studio and never gets moved, even though it was often played at high levels.........just a couple of factors "in the mix" to consider.