Like any thermionic vacuum tube, the cathode (filament) wears out and emission drops. Tubes get weak, CRTs and VFDs get dim, it's all the same mechanism. With the general demise of the CRT, the magnetron stands as the last ubiquitous domestic vacuum tube.
As with semiconductors, purity is a big player in the performance of thermionic tubes. The very best quality were used in undersea telephone cables: they boasted several decades of
uninterrupted, continuous, maintenance-free operation, in multiple cables! They were made with very high purity materials, exacting tolerances, and used lots of precious metals (gold).
The outward differences between industrial and commercial tubes are slight: usually some additional support rods for strength, or more robust materials in the most demanding types (like using hard borosilicate glass for the envelope, and ceramic wafers instead of mica sheets for support). But the big difference is what you can't see: tighter formulations and higher purity in the cathode and grid materials.
Occasionally, you'll get lucky with commercial tubes, which is why you sometimes find 12AX7s that have been in continuous service since 1960, or magnetrons as old that are still going strong. These examples are on the extreme side of the survival curve, and not at all representative of typical quality -- interpreting it as such would be a rather egregious case of survivor bias!
Tim