There's nothing to stop any of your paying customers from setting up their own site and giving away (or selling for cheaper) copies of RCU, right? I'm guessing you understand that and figure the downside risk is low--small numbers, low stakes, consumer focused, good vibes?
That's right, and they have, but I still earn enough income to develop and maintain the program. Free redistribution does not hurt my sales because people who would have paid for the program do, and those who wouldn't can end up trying it, a fraction of whom go on to become customers because they want updates and support from the most-trusted source (me).
You'll see a very similar thing elsewhere: illicit file sharing increases box office revenue and increases album sales.
Anyone who wants a no-fee copy of RCU can find it many places by searching. If someone ever extends my program with a cool new feature, their version must be released as AGPLv3+, and that means I can re-incorporate their improvements into my own offering. The market dynamics of free/libre software are such that since I have the reputation, the first-mover advantage, and the top search engine rankings, then anyone looking to compete needs to do so on quality of features -- which I have the license to copy, just as they copied from me. Anyone who wants to compete with me needs to compete with all that.
Anyone who shares my program ends up promoting it in the best way possible, by talking about it and telling others it's good and worth using.
