Our company, Codeup, is coming up on its fifth anniversary. When we started, I had a one-page business plan and pitched my two cofounders. I'd handle the business aspect and they would teach the first cohort.
Their response was: "We'll start writing curriculum immediately!"
While I admire anyone with that mentality, I said:
"Let's run some tests and see if anyone actually wants to buy what we envision."
I went out and marketed the thing. A few months later, we'd gotten deposits from enough students to know we'd fill our first cohort. Only then did we build out the curriculum.
I recommend this same path for startups. Too many build before they truly understand whether customers even care. The best way to learn that is to try to sell them and get real money to prove it.
I talk with people as to what's more important: sales/marketing or product. Early in my career when I was on the tech side, I'd clearly say product matters the most. Now I know that sales and marketing are the key factors that make or break a company.