What does follow is that apps should have a library available which allows them to express their positioning desires while accounting for the complications and corner cases.
Even if positioning were centralized, apps should still have been able to tell the WM "This is what I want to happen w.r.t. positioning, try to accommodate me".
1. Assign responsibility to the wm (Wayland’s way)
2. Create a new way to specify position preference in a complicated world. That’s what you propose, IIUC. It’s possible but complicated, and Wayland had a lot of other problems to worry about, therefore I understand why they didn’t choose this way.
3.Leave the simple protocol in place, but let the wm override the application’s choice when necessary. I guess a lot of apps would not work properly in such a system.
An exaggeration of course, but I have seen plenty of apps try to do "smart" things about window management on their own, and they always get it wrong. It's one of the reason's KDE WM allows you to to override and make permanent a surprising number of settings.
(I also believe that websites should not be able to just do whatever the hell they want on my computer, but that battle seems to have been lost.)