Using chromium as a base, browsers will have to differentiate by offering a better product.
“Better” is less likely to be better performance, compatibility, or accessibility.
By using much of the same code as another browser implementer, any browser vendor [hint hint] that still makes their own engine could reduce the resources they put on the foundations and web platform and put more of it on the product itself.
Perhaps we’ll have more groundbreaking innovations to move browsers forward. The last few big ones: multi-process architecture, tabs.
In effect, by using the same foundations, the browser wars could in fact be reignited and the users could be the winners.
On the web platform side, I.e., the stuff you see on MDN and w3c specs, using the same “base” doesn’t mean the browsers won’t have different implementations of future APIs if the vendors’ opinions diverge strongly. Case in point: Chromium used to use WebKit as its renderer and now uses blink, a fork of WebKit.