Re: Is anyone really going to use this?
The mentioned big players were, in a not so distant past, small players as well, you know.
And they all share a similar approach to building web content, which is by leveraging html, js and css. Most of these 'combinations' are used in the real world by one of a dozen existing frameworks, like the ones you mentioned, which are the most popular ones.
WebAssembly on the other hand is a totally new approach in providing content for web browsers and offers additional possibilities, like an entire 'native' DOOM game running in wasm, in the browser, and this is at near-native speed, not like the JS-based rewrite that exists.
Microsoft is not at all the only one providing wasm-support for their developers (in the form of Blazor).
I know the Java-world already has various projects in different stages of completion to offer exactly the same to their community, and I'm sure that Rust and other environments have too.
And the reason they do this is that now, for most business applications, most businesses have the option to hire 'full-stack' engineers, which need to know C#/Java/... and need to know one of the popular and ever-changing web (javascript) frameworks. The other option they have is to hire additional front-end developers which should only be bothered by these front-end technologies.
In practice this means a not always flawless or easy integration of front- and back-ends, and mostly separate codebases with developers not really knowing the others' codebase (or used technology).
When having developers that can do both back- and front-end in the same language and technology stack, and they can easily share their datatypes, things can be easier and faster, and be inside one and the same codebase.
Currently, existing proven JS-libraries can be used from wasm/blazor, but the idea is that most of most popular JS-libraries will be offered as a blazor/wasm library so there really is little reason to needing to dive in JS. An effort has already started to transfer such libraries and components to Blazor and if I'm not mistaken, quite a lot already are available.
Sure, there still will be web-applications that might be better developed with existing web frameworks, but majority of business-oriented apps will benefit from wasm.
On top of all this, when using Blazor, eventually in conjunction with the UNO-platform or Xamarin, you can provide web applications, (native) mobile apps, electron apps and winforms/desktop apps all from one and the same code.
And since most business are really going for a multi-level presence, I'm pretty sure this will surely be a hit.
A comparison with Silverlight really is not relevant here, since it was a MS-only technology, mainly supported by IE. Wasm is a technology already supported by ALL popular web browsers and is an open standard.