1. Buddy where had you hid this thread for 4 long years?
Don't know How I missed it.
BTW, let me congratulate you on a very good write up in simple language. Its a perfect guide for first timer.
2. A small correction. HID bulb does not have any solid metal filament.
There are Metal electrodes with an airgap in between. Also called Arc gap.
The ballast produces high voltage which strikes very brightly Spark/Arc across these.
This Arc is in the UV spectrum initially. But it starts to Vaporize the Metal Halides (Salts) inside the arc chamber. Sometimes you can see the halides as red/brownish or grey powder or dots inside the arc chamber. The quality and composition of these halides are what that generate visible light. The composition of Halides can be altered to produce different colors and qualities(CRI). HID lamps are also frequently referred to as METAL HALIDE lamps in industrial word.
Over the time of the the bulb, these salts deteriorate, which means they fail to convert Blue-UV spectrum to visible light as effectively. Hence you can see that older bulbs often shift over to bluish before failing. A 4300k Bulb may start looking like 4600K to 4700k
Other reason is that Arc gap widens due to electrodes wearing out. Once the threshold is reached that ballast cannot produce enough voltage to strike/sustain the arc over wider airgap. Bulbs of these age often start "Cycling" as in going on and off (restarting) every few minutes. You can see this in Ill maintained Municipal streetlights also.