In PostgreSQL, you can set the SERIALIZABLE isolation level for the whole database, put a retry loop around all database transactions and completely stop worrying about concurrency issues.
I think all database applications should be written like this at least until performance starts being an issue.
Entirely depends on the the type of workload / application architecture. In some cases the rollback ratios will be massive (I've seen 70%). In other cases adding such a retry loop is unattractive because the latency jitter. Or retaining all involved data for a retry is unattrictive.
Yet Another problem is that you need to enforce all sessions potentially involved in a data race need to use serializable; that can be easy or hard, depending on the scenario.
I think all database applications should be written like this at least until performance starts being an issue.