Quote:

...HID light output reduces and the colour changes as the bulb gets older. This is caused not just by the molecules of gases and salts inside ageing but they also defuse progressively through the glass.

https://www.motoringassist.com/news/why-yo...ight-bulbs

So the quality & thickness of the glass is going to be important, and also the build quality where the glass is sealed so that it doesn't leak there either.

You learn something every day

The advice to change them every 5 years rather implies that they should still be working at five years old. Complete failure at nine months is .