(Obviously) I don't think asking why now is the right time for your idea is premature. I think it's orthogonal to the product building piece. I think a lot of YC companies could avoid a lot of pain if they asked this question early on, because it would lead them to talk to others who have worked in the space. Often they will quickly discover that the fundamental dynamics of an industry haven't changed yet and no matter how much time they spend building it's not worth their time yet. At the very least they should be able to point to why the other, previous attempts weren't quite right and why the time is now for this idea to take hold. But all too often companies just start building without understanding why the previous attempts have failed.