It took WebGL about 4 years to spread, the reason it took so long was in hardware (GPU, drivers ...). Wasm is just CPU-related, I believe it can reach most of the people within a year.
Pouplar browsers get updated quite often. And those, which got updated rarely, became unpopular. Everything will be fine, I think :)
90% of the time someone puts a link to some WebGL demo on HN or Reddit I either don't get it running at all, or get single digit FPS after the mobile turned into oven mode.
There will be a polyfill for unsupported browsers. It should be easy to convert most, if not all wasm to asm.js, which can be executed by any browser with typed arrays
Nearly every browser is "evergreen" these days, meaning they auto-update. Chrome, Firefox, and Edge all update often and automatically. Mobile browsers also have this benefit due to being distributed via an app store. This percentage is always going up, but even now is a significant percent (the exact number I'm not sure).
What this means is that web standards can move significantly faster than in the past. Interesting new technologies like WASM can be implemented and tested without waiting 5-6 years anymore. Very exciting to see as a web developer.
I think the question we all should really be asking is how long will it take to get to the point where WASM is practically useful for anything that noticeable chunk of web users cares about.
These days software is mostly auto-updating so at least in the US and other first world countries, relatively soon. The rest of the world is where we'll be lacking. :/
