I always plug my laptop into one or two external displays.

Even without configuring distinct DPIs per monitor that was not a problem for me, because on the small screen of the laptop I kept only some less important application, like the e-mail program, while working on the bigger external displays, so I had no reason to move windows between the small screen of the laptop and the bigger external displays.

But like I said, setting a different DPI value for each monitor has been added to X11 many years ago, I do not remember how many.

I do not see why one would want to move windows between the external displays and the laptop, when you have connected external displays, so I consider this a niche use case, i.e. moving windows between small screens and big screens. I agree with you that having simultaneously big screens and small screens is not niche, so I was not referring to this.

Without a per-screen DPI value you cannot control the ratio between the sizes of a window when is moved between the big screen and the small screen, but even when you control the ratio, moving windows between screens of different sizes does not work well because you must choose some compromise, e.g. if you keep the same physical size some windows from the big screen will not fit on the small screen and if you make the windows occupy the same fraction of the screen size they will change their sizes during moving and they will be more difficult to use on the small screen.

But like I have said, this no longer matters as the problem has been solved even for this niche use case. I do not even remember if this problem still existed by the time when Wayland became usable.