Do I think that this technique can be refined? For sure. :)
One of the reasons for publishing this and being open about how this worked is to be a catalyst for a conversation around this, which would hopefully see a refinement of the technique.
I applaud you for being so open and transparent. Although I personally feel that the line was crossed, your openness to see a refinement in technique is refreshing. There are just too many "entrepreneurs" these days justifying their practices by saying that it works and it's completely legal. For me there are 3 responsibilities (in the same order) to any entrepreneur/business:
1. ethical responsibility
2. economical responsibility
3. legal responsibility
To say that something is completely legal does not make it unethical. So I do hope we can find a better way. The false advertising that companies put out makes me sad but it's also an opportunity for businesses to be honest with their customers.
Question for you though: You mentioned you received numerous signups signalling their interest. Could you have conducted your interviews with them instead? From the story, it seemed to me like the key point was identifying the needs of the customers. Of course, the ones who placed their credit card information are the ones most interested which filters them out from the rest.
Sure, I could have just done customer interviews without requiring CC details. I truly believe that this feedback is however still hit-or-miss and we might've optimized for the wrong thing thereafter. Paying customers' feedback is always better than the feedback from any kind of "free user".
One of the reasons for publishing this and being open about how this worked is to be a catalyst for a conversation around this, which would hopefully see a refinement of the technique.