It's far from cynical to ask how a business proposes to make money, especially if it's non-obvious! Yes, there are other challenges beside this, marketing, engineering, and others, but the whole purpose of a for-profit company is to make money. Asking how a business plans to do so isn't naysaying, it's a sensible question.

Also, a valuation is more or less a made up number that suited the VCs and others. The words, "okay, we'll change the valuation to..." seem to be uttered quite commonly when the previous valuation is inexpedient.