Probably depends on definition …. as in … does piano mean acoustic piano? Or digital 'piano'? And are 'digital pianos' formally counted as a piano? I myself count digital pianos ….. the classical ones as well as the 'hybrids' as being pianos. But will be interesting to find out if there is some world agreement that a digital 'piano' is officially a type of piano.
Also … I don't see music stores having less acoustic and digital pianos for sale in our city here. And doesn't appear to be less people taking piano music lessons.
Whether there is decline or not … for me …. it's care factor zero. Just as long they keep making available one or both types of these 'pianos' for sale, then that's fine with me.
For those people …. whoever they are .. as in where ever that piano is dead quote came from. They should take a good look at the pop music etc scene. Music isn't ever going away. And pianos aren't going away unless some world catastrophe comes up …. which hopefully won't occur anytime soon.