There will be less incentive for developers to make sure their stuff works on anything but Chrome forcing safari devs to just play catch up all the time.
It would take more than just being competitive, the WebKit team would need to start perfectly reproducing Blink behaviors and quirks. Devs are already more than happy to never test their work against anything that’s not Chrome and that’s only going to intensify when they don’t need to care about WebKit any more — any deviance from Chrome even if it’s technically within spec will be considered an error.
That’s where users feel forced to change browsers, because if they don’t they’ll frequently be running into pages that are broken in browsers that aren’t Blink-based.
Different even if just as or more shitty.