I think the problem there, is that you then basically wind up with Git on the backend and an SVN-like layer over the front. Which seems a bit silly. SVN is a very robust project that works really well. Why not add git-like porcelain to SVN if you're gonna force devs to have an network connection so they don't have to download a giant git working directory.

Smart people are working on all this, so I'm sure there are reasons, but in the all the instances where I've had to interact with a monorepo, it was because the tech debt was too high to pay off to break it apart, not because it was better.

And if you're indebted to the point where you have no point in paying it off, you damn well better have leveraged assets against it (i.e. a cash cow of a business, like MS Word or Facebook)