This is not really a useful response.
The trivial counterexample is that all modern crypto techniques rely on keeping a key, or part of a key, secret. That's security through obscurity, and you've just stated bluntly that obscurity never works under any circumstances, right?
What you want to do instead is talk about tradeoffs. Talk about how much information you need to keep secret in exchange for a given window of effectiveness, and state a preference for systems which provide longer windows of effectiveness while requiring less information (such as only a key, or part of a key, instead of a key and an algorithm) to be kept secret.
Also, take care with your argument about "pre-modern crypto techniques". Some of them remained effective for centuries after being invented, which is a far cry from your "cannot be argued that they worked", and not necessarily a favorable comparison with many modern techniques, which are lucky if they make it a couple decades before being broken.
(also, of course, all cryptographic systems eventually get broken, which is why every so often we switch to new algorithms, longer keys, etc., and you seem to be arguing that any system which eventually gets broken is a system which never worked, and that's also wrong)