Hi Fredrik
Thank you for finding this article on the Double Diamond insightful! To answer your question, which is an excellent question, and one of the most important questions to ask, I would say that research is the only way to understand whether people would be willing to buy a feature/product that is in the planning.
The Double Diamond's first phase is all about research. Research is an intricate and extensive phase. The basic law of UX is the more research you can undertake before starting a design, the better.
Only through research can we really understand our user base, users' preferences, needs, pain points, wishes, goals, aspirations, and so on.
I cannot stress enough to access the research tools available. Research can be done with user interviews, observations, data gathering, and so on.
I've outlined the differences of primary & secondary, qualitative & quantitative research in this article. This might be of interest to you.
Hope this can give you more information on your query.
