Serializable by default doesn't seem feasible when you really dive into the concept.

"Serializable" is a system property that describes how two or more transactions will take effect. In this context, I would define "transaction" as a business activity with a clear beginning, middle & end and exhibiting specific, predictable data dependencies. Without any knowledge of the transaction type(s) and their semantics per the business domain, it would be impossible to make assumptions about logical ordering of anything.

SQLite is the closest thing to what you are asking for. All writes are serialized by default, but this is probably not what you really want. We can ensure multiple concurrent connections don't corrupt the data files, but we aren't achieving anything in business terms with this.