Others have made excellent comments here. I want to add this:
- Mention that unless it's for a certain price range, you're no longer in the market. However, you will keep them in touch when you're changing the policy again.
- If the pricing/arrangement doesn't work for both parties, say you're happy to refer them to someone else who could use those projects.
Also, stay in touch with them regularly, but you don't need to go overboard on this, as you want to focus on your company.
Have you tried testing your idea without coding or hiring people? Most ideas in start-up founders' heads usually aren't any good in terms of making money. It's not until the pivots when they found profitable models.
You've mentioned you've talked to over 20 people and they said they would buy your product. However, getting them to pull out their wallets when you actually have the product could be a different endeavor. Not sure if the problems you're solving are painful enough.
Stop thinking of your ideas. Just pick a niche industry, and see what problems THEY are having. Only start building the solution when you see a convergence of the same problems.
Re: rdl's comment, I'm not sure. I'm not experienced with YCombinator process or startups, but can't the founders explain their application was sent as of the submission date (weeks or days before), and then explain during the interview about the important changes? I mean, Parkinson's Law is in play with entrepreneurship big time.
What kind of application is it? Promoting it in front of niche communities and giving them benefits to share your app would go a long way in terms of building your own community.
Find some potential VCs. Research the hell outta them to find their interests and what makes them tick. Then, when the opportunity comes to provide them value, be proactive about it. Give it time to build relationship until you can meet in person and go from there.
Alternatively, you can fly out to different conferences in order to meet them in person to kickstart the relationship.
You mentioned the need to develop concrete examples showing how rewriting copy is worthwhile. However, even if you're starting out for free, you won't have examples to show for it. How appropriate do you think it is to show other people's examples before doing for free to gather your own examples?