A third implementation and also having a WebGPU.framework built-in and shipped with the OS (like all other first party frameworks) would simplify the build and debug process.
That means easier builds for the developer, smaller apps, receiving implicit upgrades in lockstep with the OS. Take the example of new metal versions enabling new optimization avenues. Bundling your own dependency would mean that you need to keep on top of the changes to take advantage of it , but with a system framework you’d get it whenever Apple’s engineers enable it.