Open source contributions could ease development cost a bit, but cost isn't the main concern. The concern is compatibility, security, and performance which almost no amount of resources could get Edge up to par.

However, a radically new browser engine like Mozilla's Servo has a chance at performing so well, and reducing security vulnerabilities, that it seems like the only viable hail mary left for surpassing Chromium.

Also, these options aren't mutually exclusive. They could've decided to open source Edge but also switch to Chromium.

It would be interesting to have the code for Edge.