It doesn't lose the ability to be tuned--the state of the piano merely drifts over time if it is not maintained--imagine your teeth drifting in your gums without a retainer, for example. If it spends 80 years warping a certain way, it will just take another 80 years of careful tuning to warp it back in a gentle, safe way. Most objects are not lucky enough to get 80 years of human attention after 80 years of inattention (stradivarius violins are a notable exception--these violins stay in tune just fine, despite being over 300 years old--they are nice enough that people have decided to keep tuning them).