And if big web properties like Youtube, GMaps and Gmail etc. don't work properly, are slow or missing features on Gecko then the dominance of Chrome will increase further anyway.

The market of web browsers has become "who gets to set the default search engine makes the moolah". Bing marketshare and revenue is driven by how well Edge works and how it can stop the bleeding to Chrome. Changing the engine to Quantum does not solve the problem they were having in the first place very well.