Cathode emission decays over time, from a variety of causes; simiarly, phosphors experience burn-in.
Some of the causes (for both materials) can be considerably reduced, by careful attention to chemical purity, high vacuum, and stable operating conditions. A commercial grade vacuum tube might last 5000 hours at ratings; on the other hand, the best tubes ever made lasted decades in service, and I don't mean as special cases (survivor bias, of course your 12AX7 from half a century ago still works, all of them did, obviously
), but as true statistical samples, with hundreds of units in service at a time, and no failures over decades of continuous operation.
What you'll find in a real unit, in practice, varies. Most scopes were of the "commercial" to "premium quality" grade, with some exceptions here and there.
For example, my Tek 475 lasted over three decades of light use, then about five years of heavy use once it got into my hands. Tek 475s are known for running the heater too hot, consequently the cathode life is unusually short. I purchased and installed a replacement CRT, and it's been fine since. (I also haven't been using it nearly as much, since I've moved on to other scopes for primary use.)
Toshiba? Probably commercial quality? So, depends on how much use it's had.
Tim