Just how bad can an Ion burn be?

[Zenith26kc20]

This may answer why 7JP4 tubes were not rebuilt in the past (from what I have learned). If the phosphor is dead, it may be too costly to recoat on what, at the time was becoming an obsolete tube.
I guess I'm lucky with my 10 and 12 inch tubes. I see no ion burn on any, even the really high hour.
Would a double ion trap vs a single make a burn difference?