If there's a way to do zero-copy message passing in web workers (or equivalently, overhead-free transfer of object ownership between threads), shared memory may be unnecessary.
That helps, but it still means you can only have the data on one thread at a time. What would be a massive help would be if you could split a buffer into non-overlapping views, and transfer each view to a separate worker. Some algorithms would still be challenging or impossible to implement this way, like parallel prefix sum algorithms, but it would still greatly widen the number of things you could do in parallel.
Yes, at some layer of abstraction there will be the possibility of sharing memory. It's probably better to keep it behind the scenes for all but the most CPU intensive uses.
