While I am definitely sad that Microsoft has decided to exit the HTML rendering engine game, and even more sad that they picked Chromium, I can understand their business reasons. Likely the right move for them.
That said, here's my potentially controversial view on the state of web browsers: a rendering engine monoculture is less of a problem for users than the fact that web browser user experience has completely stagnated.
Someone needs to take a truly fresh approach to web browser UX. Separation of work/personal identities, "project" spaces with collections of windows/tabs, new models for organization beyond tabs (they scale terribly), privacy toggles, more!