It sounds to me like cfinke made a poor choice when acquiring a used instrument, then extrapolated that to 'all old instruments are bad/can't properly hold pitch'. (/exaggeration).
cfinke should've just gotten someone who knows about pianos to help him/her choose a good one, the way you might with a used car (although the choice is more subtle and less statistics-based).
As a counter example, I practice every day for hours on a baby grand piano from the 1930's which is very sturdy, with a beautiful tone. It was sitting unused (and untuned!) for decades in an old woman's house.
It was purchased for 1,000 euros at an auction, one of only two bidders.
I had it tuned by someone: it still sounded problematic, but good for practicing. I had a different tuner come and he made it sound incredibly good after a single tuning.
So the technician makes a huge difference too.
Piano restoration is fascinating, but engineers tend to underestimate how important trained hearing is in in order to improve the sound.
(rather than just being able to make a piano function properly on a mechanical level).