I can see how C/C++ (and, well, more likely Rust) will become a de-facto solution for when you need performance or some type of robustness to your apps (ie probably the right thing for Figma). But the potential performance improvement might be negligible on most makes, and unlikely to offset the fact that you'd be working off a completely different stack making things like debugging and inspecting much harder.
What I think is likely is that using WebAssembly will allow new types of web apps to become popular. Things like video encoders, image editors, etc. Things that you just wouldn't do with JS anyway, at least not in the heavy parts. Thing of a standard web app that uses ffmpeg on the background.
But in 2017 startup and companies are starting to understand how fast response times (on a website/web-app) are of critical importance for gaining a good user base.
A C solution might make the former faster, but it'd still be a dumb architecture. A cheaper solution than migrating your whole codebase to a separate language and build process is knowing how to deal with events and how to parallelize (or delegate) stuff correctly.
Not only you but many, many programmers out there in other languages, wishing for more performance, or for a higher level of abstraction. I will brace myself for the amazing applications that will be built now for the web.