If you compare a piano with a standard keyboard, there are a lot of differences. The problem with the standard keyboard is, that for touch-typing, the hands need to be angled very unnaturally, as they have to be close together, yet lined up in an almost parallel position. The only way to achive that is to flex your wrists, and that is where a lot of stress originates from.

A piano is very wide, and you often have you hands spaced very far from each other. This reduces a lot of stress. Also, you don't have your fingers basically tied to the same positions, but during the play, the wrists change position a lot, so there is no static pressure into a single unnatural position. The active movement should be beneficial to your wrists too. Finally, the keys are pretty large, giving more possible hand positions to hit them.

The piano keyboard is pretty poorly designed from a pure user interface standpoint; its design is mostly dictated by the direct mechanical connection to a big bunch of parallel strings.

Tons of pianists get RSI, which can seriously impact their careers. It is important for serious musicians to figure out a piano technique which does not require putting too much static load on any of the joints.

An improved piano keyboard could help quite a bit, but it’s pretty hard to change piano keyboard layouts, since there are several centuries of repertoire built up around the current one, and pianos are very large, expensive, and difficult to modify.

I would also imagine manufacturing a piano keyboard is much harder, since it's not a simple on/off switch like computer keyboards. They need to have very precise tactile feel, and support multiple steps of pressure and speed when pressing the keys.

It's also an un-chorded interface (pun intended) for traditional pianos. So I'm not really sure how much better it could be made.

If you used hexagonal keys instead of long rectangles, made the keys a bit smaller, arranged them in several rows but not so many columns (so they were all accessible without excessive arm reaching), staggered their heights to make pressing combinations of them as convenient as possible, split the piano into two pieces and tilted them 30–50° upward toward the center (with the idea that each hand stayed on its own side), you’d get something much more comfortable to play and more flexible as an input device. Optimizing the layout would take a considerable amount of research / feedback from musicians.

However, it is likely that some parts of the existing piano repertoire would become significantly harder to play, or at the very least everything would require dramatically different motions vs. the current piano, requiring any aspiring player to completely re-learn how to play on the new design. But in trade many new types of note combinations would become dramatically easier.

I can’t see pianists who play e.g. 18–19th century piano pieces taking this up in significant numbers, but it would be great for people developing new music.

Even the Jankó keyboard never saw any significant adoption, and that is a much less radical and more obvious improvement of the previous design. https://en.wikipedia.org/wiki/Jank%C3%B3_keyboard

You mean like the various thummers?

E.g. https://www.youtube.com/watch?v=WPxrTUnb0Iw

or https://www.youtube.com/playlist?list=PLow-C_8Eq_u5Byk-FoXi9...

or https://www.youtube.com/watch?v=kx7FFwgqUqA

From that Wikipedia link, you probably already know that several isomorphic layouts exist.