This is probably going to get a bit "rant-ey", it's not directed at you, but at the ecosystem at large (and kind of at myself).
Everyone keeps saying this, but nobody is actually doing anything about it. People (myself included) talk about how a monoculture is bad, but we still don't use Edge. We talk about how more competition in the browser space is a good thing, but we still complain when the latest features or security benefits are missing from a browser. And then when the feature lands, nobody cares.
We complain about how bloated and slow browsers are getting, but completely ignore those which are/were slimming that down. And we talk about how Chrome and V8 are taking over the world, but we won't use tech like node-chakracore[1] despite it being a very positive experience when I last used it.
I'm assuming that Microsoft is spending a shitload of time and money on their browser engine, and it was still getting very little usage. They have great performance, some really cool security features, lots of great user-oriented features, and it very rarely gave me issues as a developer, but still nobody (again, myself included) used it. Can you blame them for not wanting to continue?
I feel the same as you, this sucks. But I can't blame anyone but myself, because I don't use Edge (despite using Windows as my daily-driver), and I most likely won't switch to it, and it has very little to do with its capabilities as a browser, and more to do with network effects (i'm used to chrome, my information is in chrome, I'm used to the dev tools in chrome, and I personally and selfishly have no reason to switch to Edge, even if it's technically better)
You can't blame the company for not wanting to continue pouring money into something which is getting very little usage or return on investment at all. At least by working with Chromium more people are going to get the benefits of their work.
[1] https://github.com/nodejs/node-chakracore