As a fan of the Show I disagree. Keeping it on PS4 and Xbone at this point is holding back otherwise meaningful upgrades to the series. But I also don't give two rips about online play.
PS4 and Xbox aren't really holding MLB The Show back.
That's an old way building games, where horsepower demanded (and encouraged) totally rebuilt engines to harness new generations of power. A lot of graphical technology today is additive or replaceable, and a baseball videogame really isn't going to have much that would utilize advanced physics or virtual geometry or other hardware-intensive, transformative engine features. It's frustrating that the graphics only improved so much with the PS5 release and that there are still bugs or other issues which have sat in there for years, but making the game look better (besides the requisite upgrades of framerate and resolution and detail layers and cloth simulation and reflections) or play better would mostly require a bigger budget, not a bigger console.