I agree that it's hard to set up. I don't agree that it's hard to keep it running.
You talk about problems when you rebuild the machine, or switch distributions, or upgrade to a new version of MythTV. It's true, those are troublesome. Try to avoid doing those things!
If you want a MythTV system that works reliably, then build a Myth box, get it into a working state, and then *stop tinkering with it*.
Obviously as geeks this is hard for us to do - the temptation to upgrade everything to the latest version is great! But, if you want it to behave like an appliance, I think you need to treat it like an appliance, and leave it alone.
Of course, it would be nice if all the upgrades worked perfectly, but my main point is that I don't think it's fair to say "the overhead of keeping it running is high", if you want to include regular software, OS, and hardware upgrades as part of "keeping it running".