Since nobody to my knowledge is manufacturing or rebuilding CRTs anywhere in the world at this point what is out there is what we have and I suspect they are never coming back. Operating at excessively high brightness or contrast can kill it a lot faster, double the beam current means very roughly 1/4th the life.
In most cases a CRT that is not abused is good for around 30,000-50,000 hours of operation before it is worn out. A tube can get gassy due to a tiny leak or outgassing of internal parts. You can get a short between elements, heater to cathode is the most common but others can happen. Heater can go open, occasionally the connection to one of the other elements can go open. Then there are other, often more sudden faults. The phosphor has a half life and becomes progressively dimmer with use, screen burn is a localized effect of this but the whole face will gradually darken with use. You can also have a type of wear that affects cutoff, the more heavily used central portion of the cathode wears more than the edge which makes it impossible to get a nice brightness gradient.
The most common way they wear out is the coating on the cathode wears and emission drops which leads to a dim and often blurry picture. CRTs have a finite lifespan and a number of different failure modes.
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