...I'm gonna guess that a 100 year old piano would need most, if not all, of it's original moving parts replaced to be in tip-top shape. I don't think the economics of it make sense, ...
Yeah - that's my understanding as well. Not a really good comparison to fiddles, and I'm far from expert, but I've never heard of an old piano being rebuilt due to the sound quality of the old wood - case or soundboard. Seems to be done mostly if the old cabinet is especially ornate, exotic wood, or if the instrument has significant provenance.
I will say that the time I tried out a 110 year old Hamburg Steinway I really, really wanted to take it home with me - despite the fact that I had no place for it and I don't even play piano more than an hour or two total in a year. What a beautiful instrument!
Had it been rebuilt? Was the beauty in the sound or appearance - or both?
One final observation - it is not at all uncommon for owners of mid-lower level spinets and uprights to have to pay someone to haul them away. They simply have no value - certainly not worth the costs/hassle of moving them. Right now, my neighbors have one in their living room that you can have if you show up w/ a van to haul it away...