A piano staying perfectly in tune for 2 years is not scientifically possible.
However, the difference between perfectly in tune and acceptable in tune is a difference that is not apparent to nearly all non-piano-tuners.
A piano can easily retain a playable temperament over many years, given the right surroundings in regards to humidity, temperature range etc. and with a decent pinblock.
The same piano in a less perfect environment will doubtless go much worse out of tune, if the factors contributing to the piano holding its tuning are less than satisfactory.
As for Steinway uprights... I've found in my experience as a tuner, they are rather troublesome to tune. Then again, so are all uprights in comparison to grands. A lot of the Steinway upright tuning phenomenon is overplayed, though, in my opinion; it's an upright piano, and any respectable tuner must deal with troubles as and when they pop up. When called to tune a Steinway upright, I don't shy away, and I don't see why other people do. After all, it's a piano...