When figuring the "lifetime" of a HeNe you not only can expect the 15K hours, but it doesn't "burn out" at the end of those hours. That estimate is the "working" lifetime. As the tube ages it VERY s l o w l y begins to lose output.
Most of the tubes we get as used (and then use them for another lifetime) were pulled from equipment AFTER the rated life was "used." When the HeNe's output drops below the OEM required output in the machine, they pull them, replace them, and sell the pulled unit to a surplus/salvage Co. or just give them away to employees.
This is one of the main reasons why most of the tubes we get on eBay, (etc.) put out well below their "rated" output.
So, if after five years, your lamp loses a MW or two AT THE BEAM, will your lamp be trash? Nope. You will still see the spot AND the plasma will still be just as bright. Even after the gas mixture gets so depleted it will no longer lase (no "spot/beam"), the plasma will still be going strong. The "hue" of the plasma will shift toward oxygen, but it will still glow brightly.
Try THAT with a diode LOL!
Burn baby burn
Peace,
dave