The Studio Display has 4 ports on it, a single input and 3 downstream ports. To use it with more than one computer, you need a very expensive thunderbolt KVM or just manually swap cables.
And yes, the multiple inputs and ability to have basic usage/control with a non-Mac is a deal-breaker for a great many people. Many more people would buy this monitor, and likely even people without a Mac, than people who would buy a Mac specifically to use this monitor. Even people completely in the Apple ecosystem could use a second input, those with a desktop Mac and would like to dock their Macbook.
I think there's a big difference it marketing aims worth noting here. Let's compare Apple to Dell.
Dell makes an extensive line of displays which they are happy to sell you whether you want it for a PC, Mac, Linux-based PC or gaming console. And if you are a PC user, they're happy to sell you a Dell Display whether your Windows PC is a Dell brand PC or one of the myriad other brands. Dell doesn't visibly care; they want to sell a Dell display to anybody willing to buy one. Yes, they sell Windows PCs and would probably like it if you also bought a Dell PC, but I see no pressure to do so.
Apple is very unlike that. When they produce a peripheral, it seems like it's only made for an Apple ecosystem product, ignoring the rest of the computing world. The ASD was a Thunderbolt 3 input-only display with no external controls at a time that bound it tightly to Macs. We don't see Apple Pencils (Pro or otherwise) used on Android tablets. Apple Bluetooth keyboards can be using with non-Apple devices, but they don't have the functionality to remember 3 different devices and switch amongst them with a button push.
Of course, some people have more than one Mac, and at least 2, if not 3, inputs would be useful for many, and Apple ought not feel threatened by console makers, but still, the tendency to have Apple products nudge people toward the 'walled garden' Apple product ecosystem, coupled with their strongly minimalist industrial design heritage (e.g.: fewer ports on MacBooks), leads me to think they may not expand input options on the next gen. ASD.
From the user perspective, it'd be nice to have extra inputs, so you could more easily get maximum use out of your very expensive Apple display.
It's like Dell wants to sell you a peripheral and Apple wants to sell you a platform. While Dell would happily sell you a complete computer system, you can buy a Dell display for a non-Dell system and not feel like you're missing out; with Apple, if you want the maximum experience, it needs to be an Apple ecosystem experience.
Interesting question...if the ASD came with added HDMI and DisplayPort ports, how many more would've been sold? Would the PC crowd have bought a bunch? Console gamers would skip due to 60-Hz refresh rate.
