Although Sean's post raised my eyebrows too, I think it's pretty academic to focus on how a device might theoretically continue to operate to some degree when it's damaged or broken, when obviously if it doesn't sound right we're going to want to repair or replace it ASAP.
Though I'm certainly no engineer, I think many of us realize that yes, vacuum tubes can be more robust than transistors under certain unusual conditions having little-to-nothing to do with home audio (some kinds of radiation exposure, thermal stress, higher voltages), and yes, they can be made to last a pretty long time under relatively harsh conditions where transistors are at a disadvantage (in radio transmitters, microwave magnetrons).
But when it comes to home audio, I think all we tube lovers can admit that our vacuum devices will be much more likely to fail, and anyway need a hell of a lot sooner replacement, than silicon semiconductors. In fact, tubes will audibly degrade over their useful lifetime to a much greater degree than transistors. And the ways they usually fail prematurely, such as losing their vacuum, blowing like an incandescent light bulb, or getting intolerably noisy in one way or another, are going to cause us to replace them, not marvel at how they might still be able to function a little bit. (But I would think the same goes for a failing transistor - I've just never had one go bad yet). In addition, tubes are generally more sensitive to some stuff that does find its way into the audio environment, like vibration and accidental impact.
Anyway, I've never thought "graceful" when one of my tubes died, except to the extent that it didn't take out anything around it, knock on wood (and once it did). But regardless, this whole debate is irrelevent to the question asked, and I'm not quite sure why Sean brought it up in the first place, unless he meant to imply that amps using large numbers of output tubes to develop higher powers would entail discouragingly high retubing costs...