[voidmain]

> any application can be made correctly with READ COMMITTED isolation if required set of "mutexes" and their acquire/release is correctly implemented

This is true, but it's "a big if." And the result might be faster than a good implementation of serializable isolation (since the latter is working to maintain some invariants that you don't actually care about), but it could easily be much slower: very broad invariants will require very broad locks which cause much more "serialization" than is required for serializability.

Let's say you want the same sum invariant but across rows: all users' balances must sum to zero. The straightforward application of your reduction is to essentially have a global mutex for all balance transactions, but this is nowhere near the most scalable solution. Whereas a good serializable database will handle this situation scalably, safely, and automatically.