About the lifespan of CRT"S

[rca2000]

I see so many old (pre-1960) articles on how CRT's often last only a few years or so. However, personal experience tells me, that many of them will last indefinitely. I just acquired some get tech-tips, and an an article on GE's release of their 110 degree tube in 1959 or so, and how they life-tersted a batch, and found that after 2000hours of continuous use, 75% of the cathode material remained. To them that was not good enough, so they re-designed the gun, and got it to where it had 98% of the material left after the same 2000hrs. That would indicate that the tube would last 10-20,000hrs. or more. So, WHY the pessimism early on about crt life, when we all now they often last almost "forever"(with a few exceptions, which were no good from the start, like 91-99 Zenith tubes, Sony 26" tubes, predicta tubes and so on).?