I agree as to the inefficacy of software gatekeeping, but marketing is a wide subject. It isn’t merely advertising, but, as I noted above includes such business considerations as reputation, first-comer, network effect, and brand name secured by the DNS.
So the developer has to be an efficient businessman as well as an efficient coder, or partner with someone who is.
I might add that, in addition to the above factors, if the service has low fees, and is very prompt (if human) or a few seconds (if automated), then it may not be worthwhile for the knock-off artist to get involved.