Because software monocultures are bad. The vast majority of browsers are Chromium based. Since Google de-facto decides what gets in Chromium, sooner or later the downstream forks are forced to adopt their changes. Manifest V3 is a great example of this. You can only backport for so long, especially when upstream is being adversarial to your changes. We need an unaffiliated engine that corrects the mistakes we made with KHTML/Webkit.

They are pushing hard on the developer experience because greenfield projects aren’t being built using Windows centric tooling anymore. If it’s server it’s Linux, and if it’s client it’s either electron or a web app. What will kill Windows is when there is no reason to buy Windows. MS recognizes this fact and has been pivoting to service offerings for that reason. They want users to make an MS account so they can herd people into their ecosystem.