I don't understand this. What are you afraid of? Are you like Richard Stallman, only downloading webpages from email clients or something? Do you have filters that strip all javascript from the web pages you browse?

If you're not one of these guys that refuse to execute any binary code that you have not compiled yourself, you should welcome the possibility of running binary code from a sandboxed web environment.

I, for one, wish I could run all my proprietary software on the web, and restrict all my native applications to FOSS. That's a prospect WASM enables.