The overhead would be terrible. If implemented, you'd put the syntax in your queries, but a GUC or some such would turn off validation, it'd be off by default (because of that overhead) and you'd only turn it on for local dev and for testing.

The underlying issue, mixing up the cardinality of one side of the join, isn't a super tricky one to identify, especially once you've seen it once or twice or seen what the buggy output looks like. Every intermediate SQL developer has run into the bug and has fixed it. The vision I have for the feature is more like a set of training wheels for developers who are earning their SQL scars. And it's also probably not a terrible idea to document exactly where you are expecting the full M x N cartesian join results when you do want them.