"As a Rails programmer, the more popular Rails is, the more worth are your Rails skills."

This is still subject to the laws of supply and demand. Your Rails skillz will be valuable at a time like now - where there are a healthy number of startups using the framework for development, and a sufficient shortage of experienced Rails devs to hire. However, if Rails became popular enough that everyone decided to learn it and the number of available jobs didn't shift in proportion, you'll find your skillz less of a commodity.

It hasn't really happened to Java, though, even though it has been popular for a while.

So how much does a Rails developer earn at the moment? Anybody want to hire me, for twice the salary of a Java developer?

That is most definitely the case now. The supply is greatly outstripped by the demand.

In what sense? I am guessing they made good money with Cobol for a while. Certainly more than if it had never become popular.