From what I can remember, I have never seen one single VFD that have managed to keep the intensity if that VFD have been used in equipment where not all segments are normally used (giving me a way to verify intensity loss without test equipment).

My microwave - selecting "calibrate" lights up new symbols way stronger than the digits showing the current time.
Same with amplifier, VCR, CD player, DVD player, ...
I have quite a number of equipment with VFD, and they all suffer from uneven segment intensities based on number of hours/day the different segments have been lit.

A 10% intensity loss is just about nothing to our eyes, since our eyes have very large gradation capability without changing pupil size of chemically changing the sensitivity of the eye. But while our eyes are very good at relative comparisons, we are truly lousy at absolute measurements. That is also why we can adapt and see "white" over a quite varying light temperature. So a 10% degradation is huge, when seeing two units besides each other, but practically nothing when seen alone. It spans too little of our dynamic range.

But of course VFD aren't alone with loss of intensity.

I have LED matrix panels that have been in use for 15-20 years, and where the top/buttom lines of LED are way stronger.

TFT panels with CCFL backlight isn't so easy to show ageing on, since they have 100% of the panel lit continuously, making the loss of intensity uniform. But very strong display panels for use in shopping centers etc do have replaceable backlighting just to allow the TFT panel to be used for more years than the backlighting. This obviously way improved with newer panels with LED backlighting.