There will always be false negatives, ideas that are rejected by angels but which later become enormously successful. However, if the industry's finest angels are all passing, and you can literally find nobody to invest, it is probably a wise decision to give up. While angels are far from being 100% accurate in choosing what to invest in, overall they have to have some sense of what's a good idea in order to sustain themselves. If they're all passing, better off hitting the drawing board.