Microsoft using Chrome only somewhat reduces Microsoft's power in this regard. In no way does that change your ability to project your will on the internet through Chrome forks. Small devs have little power now and that will stay unchanged if Edge goes Chrome.

Web devs will have to build for the lowest common denominator so this change improves that slightly.

An actual downsides is it increases the power of the Chromium team to make defacto changes.

It might also reduces innovation but I can't really see Microsoft deciding to drop a feature because it would take them further from the chrome trunk.