> the majority of concert hall pianos are going to stay imperfectly tuned.
Any "important" concert hall will tune the piano before each concert. If there is no concert for months the piano will sit untuned (but still be practiced on), but a piano tuner is cheap insurance that nothing happened. If there are two concerts on the same day they might or might not tune between them.
Of course there are a lot of "small-town" concert halls where the piano is not tuned as often. Your local high school probably isn't tuning the piano before the concert - even though the piano is moved from the choir room to the stage before the concert which is the most likely way to mess up tuning.
Any "important" concert hall will tune the piano before each concert. If there is no concert for months the piano will sit untuned (but still be practiced on), but a piano tuner is cheap insurance that nothing happened. If there are two concerts on the same day they might or might not tune between them.
Of course there are a lot of "small-town" concert halls where the piano is not tuned as often. Your local high school probably isn't tuning the piano before the concert - even though the piano is moved from the choir room to the stage before the concert which is the most likely way to mess up tuning.