It is hard to see much of a downside to Microsoft ditching EdgeHTML in favour of Chromium. Aside from a little wounded pride within the bowels of Redmond and some fanboys desperate to see Microsoft "beat" the likes of Google, many developers would be relieved to see a reduction in target platforms.
Diversity. I don't want to see Microsoft "beat" Google, but I do want to see at least a few different implementations of browser engines. Let's not forget that MS disbanded much of the IE dev team after IE6, feeling that they had "won" the browser wars. Consequently the development of web technologies stalled for at least a couple of years until Mozilla started pushing niceties like tabbed browsing in their "niche" Firefox product, and then Google stomped on everybody with the blistering V8 JS engine.
MS wanted to own the web. They imposed their own ActiveX controls and various other proprietary standards. A legacy that many organisations are still dealing with.
Google also wants to own the web, and perhaps in the past they have been less closed-shop than MS were - a benevolent dictator. But fundamentally, allowing one corporation to effectively dictate the planet's technology choices is not a good thing. The web has always worked on the principle of an organisation pushing out a new feature or protocol, and if people like it (e.g. SPDY), then it gets adopted more widely until someone eventually writes a standard (like HTTP/2). On the other hand, if it's not so popular, it gets to die a death.
But in order for that process to work, you need a community. Multiple browsers, hopefully running multiple engines (I'm looking at the slew of Chrome-a-likes here, Vivaldi, Opera), and multiple servers at the other end.
The server side is reasonably buoyant at the moment - IIS, Apache, nginx, Litespeed, GWS, et al. The browser side not so much. There's no shortage of browsers touting new user-side features or interfaces, but they're all just a glossy skin on top of Chromium...