It's a great problem to have. I think many of us here are in a similar position. Nice comfy jobs, well paid with no chance of promotion. I've been at mine for over a decade. I know it's going to be bad when its over, however there is also the real possibility of this position lasting until I retire.

I think you may be under estimating luck. Maybe you are not underemployed, maybe you are overly lucky?

If that is the case, then let it ride!