> But to ask "why even have it" is to fall into a discussion more about backwards compatibility than design.
No, it is a question about design. Window environments like Windows 11, macOS, Gnome or lots of other Linux window managers are designed for desktop PCs, not tablets. I would include Laptops under desktop PCs for this discussion, since most people -- that are working the whole day on their laptop -- add one or more screens to it. And on those large modern displays, it is more effective than ever to use overlapping non-fullscreened windows. Then it is absolutely necessary to have good standardized UI components to manage those windows. But those customized title bars break that standardization and customization. On modern Gnome I often have to search for the window that is in focus. It used to be clearly visible with a significant color difference in the titlebar. Today, there is slight difference in gray shading. That is ridiculous. And half of modern programs redefine the colors of their title bars, so you have to know for earch application which shade of gray stands for focus and which stands for not in focus. This is fucking stupid.
If there is a clean way to get rid of it, then sure that's a good option for some programs. (But there are a couple bits that you need to keep.)
But to ask "why even have it" is to fall into a discussion more about backwards compatibility than design.