Think of WebAssembly as a binary encoding of asm.js. You'll be able to deliver the same program in either format. Asm.js will still be around to support older browsers.
> Asm.js will still be around to support older browsers.
I don't see a need for this since WebAssembly will provide a polyfill.
The polyfill won't benefit from browser based JIT compilation. If you're concerned with performance, even for browsers that don't support wasm, you'll want to switch to asm.js.
I sort of think about it the other way round, actually.
Not sure what you mean? Asm.js was defined a couple of years before Web assembly
Yes, but it was defined after actual assembly/low level languages like BLISS. So it was defined as a way of doing something "assembly-like" (or BLISS-like, or BCPL-like) in an environment that didn't allow for such code.