Living on assumption and promises is more like spending money you haven't received. People can make promises but how sure are you that the promise will be fulfilled.
Yes, It is not easy to Live life based on assumptions and promises is a risky and unreliable way to make financial decisions because Promises may not always be fulfilled, and assumptions may not always prove true regardless of closeness factors such as friends, relatives or family and Relying on yourself doing this without considering the risks involved can lead to disastrous consequences, as the example you presented shows.
Make your business decisions simple, why? that because To avoid this risk, yes. it is important to plan and make decisions based on what is currently available, rather than relying on promises and assumptions. If it were me, I would start the business with the available capital instead of waiting for the promised funds, or avoid spending money that has not been received and I'm sure on the way there will be the best solution.