Alright, so I do computational plasma physics simulations for a living. Given our problem sizes often reach sizes which require teraflop level of parallelism to get done before I retire (that's my scale, I'm sure you're aware of petaflop sims the fluid/engineering guys do) we do need concurrency but it's done by chopping up the domain, literally the volume of space we simulate, onto processors. Now, we could possibly do async at some level (sub node level), but we absolutely need to be syncronized globally because we are simulating real-life physics and we need to maintain causality, especially when we pass information between nodes. Say a wave front passes across one node boundary into an adjacent one, waves must move at a finite and causal manner, of course, just as if you watched a ripple on the surface of a puddle.

If we did the whole problem async (async by default as I interpret it) it would require so much bookkeeping to keep causality that would make it unreasonably difficult. For things that require causality like that, we are lucky that the default mode of computation is synchronous because it fits my problem domain perfectly.

That's why I say "not everything is web" because while async maps well to problems like a server-client type application, it doesn't map well to all problems like my problem for example. Also, as someone else said, sync is easier to reason about for problems that don't require a lot of parallelism.