Given the history of the web and what I know of the people/companies behind wasm, it seems very likely that there'll be a graceful degradation/shim in the form

Given the history of the web and what I know of the people/companies behind wasm, it seems very likely that there'll be a graceful degradation/shim in the form of a wasm interpreter written in JS/asm.JS (or even a JIT from wasm to one of them), so there'll be no need to wait for all browsers to have implemented it, it just won't live up to its performance on all platforms.