[codingdave]

OK, lets roll with your example. You want to build a time machine. Cool.

What limitations will you end up with? Once it is built, what will the user experience be? Can you change history? Can you travel to the past or only the future? Can you return? It is interactive or view-only?

Without actually spending the money to do the R&D and see what you can achieve, you have no idea what price people would be willing to pay for the experience, because you cannot tell us what the experience will be. So you have no idea whether the price point would be sufficient to recover the R&D costs. Sure, it gets you thinking of how to "move the needle". But... what needle? What problem are you trying to solve? Who has that problem? Can they afford the solution?

Your example falls into the exact trap that people try to avoid by creating a quick MVP - jumping to conclusions about ideas that the founders are 100% sure people will buy, and assuming that people will pay for it, without actually doing the work to prove it.