Zitat:
Remove point 6. Why turn a customer's suspected or imaginary security concern into a real security issue for the provider/manufacturer?
Because it is is right to take care of his business, security and managing his own risk factor.
OP question can interpreted as relative subject, in general you build a solution and market it, you should show some security concerns or at least mention them, to show your professionalism, so if the software on a web site with sales contact, you have to address potential clients question to each and every question, and if fact if you have like 100 clients 5 or 10 of them refuse due security or whatever concern you can't explain, then it is fine and your business is OK, that belongs to your own risk factor of losing clients, not matter the percent.
But if you are after one client, you are the one need to convince him then you and only you should make the compromise to satisfy him, it is simple market, offer and demand.
Zitat:
You don't have to explain why Delphi software is more secure, but rather why your software is secure.
Now to explain if Delphi is better or not, this is absolutely relative, for your it is crucial, for him, it is not, and i agree explaining your software is secure is the best way to go, but how without source and audit (review or whatever) ?
Almost every company out their show their clients list as card to gain trust and establish this trust chain, for small companies or individuals, your code is your proof, then your reputation, these two things goes hand by hand, the reputation level you gain the more client you will have, then less question about your code quality before hand will be asked.