Yes, you can. There were existing high-value ($100B/$1T+) markets before either of those companies started. Focussing on SpaceX specifically, Musk has always b

Yes, you can. There were existing high-value ($100B/$1T+) markets before either of those companies started. Focussing on SpaceX specifically, Musk has always been clear that his business goal was to provide space transport at 10% the current cost. There's a story that he'd worked out (based on his existing rocket-building knowledge) that it seemed entirely practical to build a rocket for far less than going rate - on a plane while flying back from Russia. He had a business case before he spent his first dollar. There's obviously remaining uncertainty but it would be foolish to start a project without some idea of how you'd profit.