It's easier to justify removing stuff when it's very bulky and expensive. But a single HDMI port on the back of a desktop monitor would take up relatively no expense and space. HDMI has a fairly long life left yet, so much that even Apple backtracked on removing it from the macbooks. Which is far less of a problem since you can get a usb c to hdmi adapter but the other way is significantly harder.
Additional ports would complicate the user experience. The Studio Display has no buttons on it, but if you added additional inputs, you would also need to add a button for input switching at least. And potentially other buttons for brightness and volume settings.
It may not sound like a big deal, but I have an LG monitor that uses a remote for input switching and volume controls, and a BenQ monitor that uses buttons, and both provide a noticeably jankier experience.
The Studio Display provides a very clean user experience when paired with a Mac. You plug it in, it turns on, and all other functions (volume, brightness, colors, camera, etc) are controlled via MacOS. Personally I'm happy for Apple to optimize for that experience, at the cost of not working with non-Apple devices.
Sure, having two inputs does require some ui to switch inputs. That said, you could get the same user experience by simply only plugging one device in to a monitor even if it has multiple inputs.
If it was possible to use adapters, this problem would be much reduced, but as it is, it's pretty much impossible to plug in a desktop or game console in to the Apple monitors. And at least for me, having a joystick on the back of the screen for input switching is less problematic than a monitor which only works on some of my devices.
