Assuming the rumor to be true, what would it mean if Edge switched to Chromium as a render engine?
For market share, not much, see the previous section. Potentially, it could mean a tiny bump up, as Edge would become more compatible with the Chromium-dominated web and therefore attract more users as a browser in which the web “works”. I don’t think that would make a serious dent though, because it’s not a unique capability.
For developers, it’s one less browser engine to worry about, if they were worrying about it in the first place (unlikely). Less testing effort, less browser-specific bug fixing, a slight productivity boost.
For the open web: it’s complicated. If I were to put on the hat of a pragmatic developer, I fail to see the big gain in having competing browser engines. Pragmatically speaking, if I enter code and run it, I want the output to be the same, no matter the engine. Getting different results or even bugs in this output is not a gain, it is a pain. Having feature disparity between engines sucks, it means building multiple versions of the same thing. You can give it nice names like “progressive enhancement” but that doesn’t change the fact that it sucks, from a purely pragmatic productivity point of view.