Customers are very busy people too, and they are not on your payroll. Totally agree that you should make your decisions as informed as possible, i.e. do as much Customer Development as you can, but ultimately a big part of a early-stage startup is making decisions without the luxury of a complete data set i.e. gut and luck.
Which is precisely why you need to "ship early" (get paying customers ASAP) and "ship often" (start developing them immediately and incorporating their feedback into development).