[bluGill]

Plastic generally isn't an air tight seal. The leaks may be slow, but generally we hope vacuum tubes last for years.

[Supernaut]

> generally we hope vacuum tubes last for years

If I may offer an anecdote, the output stage in my guitar amplifier is powered by a GEC tube that is now 55 years old. It sounds great. When I found the tube, it had been rolling around for a couple of decades at the bottom of a wooden box.

This could simply be survivorship bias, but it does appear that back in the day, they knew how to build these things to last.