Most remarkable: "The bulb that still worked after 70 years" - "A light switch - probably installed in the 1950s - illuminated a large Osram bulb marked 'HM Government Property'."
First: This bulb was not turned on, and a light-bulb in storage does not degrade, there's no oxidisation happening inside the bulb itself.
Second: heavy filaments consume more power to emit light, and a heavy filament is the requirement to have a long-lived bulb
Third: Even with a heavy filament, bulbs will dim in their light output with years of use. The longest running light bulb in the world (101 powered on years if I'm not mistaken) is less bright than a candle, although originally it was about as bright as a 40w bulb.
More information on that bulb: https://en.wikipedia.org/wiki/Centennial_Light
Depends on the quality of the seal and how well it was evacuated. Older bulbs tend to be higher quality, thicker glass, much more material around the seal (the spot where the wires cross through from inside the bulb to outside).
https://en.m.wikipedia.org/wiki/Phoebus_cartel
I suppose it's accurate to call it a conspiracy since they literally conspired to divvy up markets and set predetermined bulb lifetimes.
The bulb actually reads: "Property of H.M. Government."