[vc8f6vVV]

Depends on the nature of commercial code and if it has another level of parallelism (think of web servers and read my comment below). As for DB queries, here's the thing: most commercial code is using DB transactions and there is no way to run transaction across multiple connections, so you are either single-threaded and do things in sequence anyway (why use async then?), or you are multi-threaded and then forget about transactions. Besides that, even if you can get away with multiple transactions there are those pesky questions like "what to do with a partially failed state?". Not all transactions are idempotent, and not all are reversible, it's hard enough when you run them sequentially, and running them in parallel and dealing with a failure might be an absolute nightmare.

[maxbond]

Most web applications (every one I've ever worked on) use connection pooling to run multiple transactions in parallel. I suppose you could think of that as a sort of network level parallelism, but it's not multithreading.

Connection pooling is of course not without it's hazards, scaling databases can be very difficult and almost all of the production incidents I've dealt with involve a database running out of a resource (often connections). But for your garden variety web app, it certainly isn't a dichotomy between serializing all concurrent updates or losing atomicity.