Chromium is an incredibly complex codebase, and they change their internal API's quite rapidly, so upgrading is a lot more difficult than you would guess. For example, this is what it took for Electron to upgrade from Chromium 69 to 70: https://github.com/electron/electron/pull/15405/commits, and Electron doesn't even make that many modifications to Chromium. If Microsoft were to make a decent number of changes, they would need a whole team of people just to import Google's changes, and if they start adding major new features I doubt they'd be able to keep the two in sync at all.
I would do it if the money was right but there is also the risk that Google may make the codebase intentionally difficult to port to the fork. I don't think such an effort would be sustainable in the long run.
It's not an easy task indeed, but it's very feasible. We have some examples of that kind. Firefox' fork PaleMoon is doing well, thanks to individual efforts of approximately one man. RedHat used to backport a ton of features to their Linux kernels across the major version numbers.
