well all your assertions are wrong. you think that there is only one database and no read only slaves. you also think that we always need strong serializability and acid. guess what? a users does not care if he needs to reload the page until his picture is online.

yes there are workloads, where everything you says is true. but most other workloads, like 80% of all the web pages don't need what you describe.

also some pages don't have a conventionell database at all. some people have a cache or some other services in place, some people use microservices, some people connect to other internet providers, other services like lpd/ipp etc. the world is just not black and white. everything what you describe is uterly crap since you just try to talk around, cause your application is not as complex as others. and yes in prolly 60-70% of the cases async will not yield more "speed"/"performance" however you call it.