They couldn't leave WPF alone to prosper... they had to "improve" it. Then the replacement wasn't good enough and they had to replace that as well. In the process they pushed product managers to say "Screw it, we'll just build the UI in a browser instead."
If anyone "kills Windows" it will have been Microsoft themselves.
The problem with WPF is that it's tied to Windows itself, which isn't of any use in today's cross-platform, .NET 5+ world. (Before you shout, yes I do know about Avalonia. It's nice. But too late.)
I still don't get the use of Catalyst to get the iOS code running on macOS instead of proper macOS bindings, like the Visual Studio for Mac team had to use for their rewrite (they aren't using MAUI).
Then the whole Blazor hybrid stuff, there is a Web browser for that, no need to make the stack even more complicated.
We have a Xamarin Forms app where I work, which due to too many other things on the docket was put off for updating to use MAUI. Now it looks like we've dodged a bullet thanks to all our other tasks, and I won't have to worry about starting that port work until MAUI's more stable and functional.
It used to be said that Microsoft v1.0 products should still be considered beta, which is fine for early adopters. However, a lot of folks who preferred a more solid product would wait for the v1.1.
Microsoft recently advanced .NET MAUI to General Availability status, but many developers have complained about half-baked functionality that was shipped too soon
If anyone "kills Windows" it will have been Microsoft themselves.