Now from a technical perspective, more diversity in browser engine might be nice, but this doesn't really contribute to the competition discussion.
(my opinions, not those of my employer)
Chromium/Blink being FOSS with Google also by far being the most dominant contributor essentially allows them to wield the same sort of power over the web that Microsoft once did, except this time it’s been whitewashed with an “open” gloss that helps keep the regulators off their backs.
I think it'd make a bit of a difference, because Microsoft would now have the threat available of changing rendering engines and taking all their users with them (via Windows automatic updates). It'd be expensive and risky, so they wouldn't really want to do it, but they'd have it as leverage against any Google decisions.
If Safari had 90% market share because it was a great browser available on every OS promoted on some of the most visited websites (which, in this thought experiment, would be owned by Apple), while people were able to choose their browser on iOS, and browser vendors were able to choose the browser engine and if a version of Safari without sync were open source, that would make a difference.
WebKit has always been and still currently is a standalone browser. WebKit even has a standalone browser version for Linux too.