alt Hacker NewsHandwaving "just expose an API" ignores the mess at the extension boundary. Modular only works if the contract is airtight, and with Wayland's churn and "sorta spec" documenation, that sounds optimistic at best.
Every "flexible" API turns into a leaky mess unless someone is paid to write the dullest test suite in existance, and nobody is. Mandating one design is ugly, but pretending composition is free is a fairy tale.
Well, window management is likely compositor-specific in the first place, so I don't really see the problem. It's not part of the spec, because it would unreasonably constrain what an implementation could do. E.g. what window management does a kiosk compositor have? What even is window placement, what if I am writing a 3D compositor so x/y positioning doesn't even make sense?