Edge isn't very portable outside of Windows, it's just the cleaned up and refactored IE Trident engine. The same one we've technically been using for two decades.
Then why does IE still exist alongside Edge?
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.
EDIT: Considering the relatively slow speed of Safari development, I would not be surprised if they also would switch their base to Blink.
Where Google favors getting new features in at a breakneck pace and sees the browser as an OS-like platform where the developer rules supreme, Apple favors power efficiency and sees the browser as a content portal where developers can’t be trusted to work in favor of users. These two models are fundamentally incompatible.