They can diverge from Chromium around the edges but they are dependent on Google for long-term Blink evolution. That includes not just the internals but also APIs. E.g. if/when Google removes powerful content-blocking APIs from Chromium, Microsoft is would struggle to maintain their own different API. Especially for APIs that have architectural implications ... converting something from sync to async or vice versa. Significant architectural divergence would get expensive pretty fast.
Then again, they would face the same issues had they adopted Gecko. Maybe less so because Mozilla could be more easily influenced than Google (in some ways). The main argument for adopting Gecko would have been that giving Google complete control over Web evolution is a threat to Microsoft and that adopting Gecko would reduce that threat. I'm a big Mozilla fan but I think it would have been a weak argument.
Then again, they would face the same issues had they adopted Gecko. Maybe less so because Mozilla could be more easily influenced than Google (in some ways). The main argument for adopting Gecko would have been that giving Google complete control over Web evolution is a threat to Microsoft and that adopting Gecko would reduce that threat. I'm a big Mozilla fan but I think it would have been a weak argument.