WebAssembly is a thing that web page Javascript can choose to use, and the Javascript is in control of what it exposes to WebAssembly. WebAssembly doesn't grant or somehow take away any privileges to the web page. (Ignoring performance, it would be possible to implement WebAssembly inside of a Javascript library as an interpreter.)

>Don't you think there is lockdown now?

No? Anyone can author and host a website, anyone can view html+js saved on their own machine, etc. Open source web browsers exist, are fully-featured, and are popular.