How long could electrode—less lamps of the Neon, argon and helium variety last?

Would there be any blackening of the outer envelope if no tungsten were present inside?

Would gas escape the glass (or quartz) somehow over time or could it be contained forever within a perfectly sealed sphere?

I suppose as long as the driver/HF supply/gear would last. I’ve got Philips’s 85watt QL lamp, and that’s just a low pressure mercury ‘jar’ that sits over an post with wire wrapped around it, (it’s antenna), I think the life of these are until the driver gives up?

With the induction lamps you also have the problem of the phosphors deteriorating..

In fact the phosphor uses to be the weakest part even in electrode based rare gas discharge tubes.

The Ne and Ar use to be sealed well, but if the lamp relies on some Penning mixture, the gas cleanup (gas atoms get attached to the internal walls) may still be a problem, as it may mess up with the exact ratio, making it harder to start. But this is usually reversible (once warmed up, the heat releases the gasses back)

Helium will definitely go away, just because the He atoms are so small it is impossible to really seal them anywhere.

By the way do not mess up with He close to any new smartphones or smart watches. These devices often use MEMS oscilators, which are extremely sensitive to He pisoning (He diffuses into the vacuum cavity with the MEMS element, damping it so much it really stops working). Although the effect is reversible (in clean air the He seeps away and the MEMS starts working again normally), with nonworking 32768Hz oscillator the firmware usually collapses and messes up so badly the device gest bricked. The MEMS are used instead of the crystals because way smaller and so can fit more easily into the tight electronic of the smart devices, plus normally use to be even more accurate and require a bit less power than quartz.
And CRTs are sensitive to He as well - it seeps in, contaminates the vacuum and causes discharges.