I've been an occasional visitor/reader on this site, and this conversation interested me enough to join in and add a couple of points. I know there's been some contention and I don't mean to stir any of that up, but a couple things are coming to mind that I don't think have been mentioned yet.
I think it's certainly not clear that a digital piano is a piano.
Point 1, consider the distinction between "electronic keyboard" and "digital piano." When I started playing in the early 2000's, it was clear that "digital piano" was marketing phrase. But even so, there were definitely some steps taken towards the electronic keyboard evolving into something more piano-like. But at what point does it stop being an electronic keyboard and become a piano? If it has a wooden cabinet, if it has weighted keys, realistic sound, etc? And if an electronic keyboard is something distinct from a real piano, then you'd have to ask: is a digital piano also a kind of electronic keyboard? I would say yes. And then, if an electronic keyboard is not a piano, neither is a digital piano.
Point 2: Five or six years ago I got a low-end Roland digital piano (FP10 I think). I was talking to someone and I told them that I had gotten a piano. Then I regretted saying that because I felt like it was a lie or an exaggeration. Now I'm an owner of a Kawai hybrid and I no longer feel like it would be a lie to tell someone that I have a piano. Maybe stretching the truth slightly, but not enough to waste any words in clarifying my meaning.
I think you can't deliberately change language, nor is what Merriam-Webster says anything more than a crude (and not necessarily accurate) summary of the language at a certain point in time. My intuition is that a digital piano is a kind of keyboard instrument.
