Sure, but they still need to build a plane, and not just any plane, but an aircraft never before built. What do you think their game plan is moving forward?
The team needed to build such a plane might be far removed from the network or knowledge needed to raise the money necessary to build such a plane.
In that case, YC is tremendously helpful in giving the platform to raise enough money to do all of the hard things involved in building a plane that's never been built.
Time is the most precious resource in startups. One way to get more time is to have a phenomenal team, so you do things faster (either by building the right things faster, having the expertise to not build the wrong things, etc.)
Another way is by having more money, so you have a longer time to figure things out. When I say that the $120k isn't what is valuable about YC, I don't mean money in general. The fiscal benefit just tends to come indirectly, from access to platforms (Demo Day, press), investors, and pitch advice.
Not immediately. Businesses that have these immense goals are about research, discovery and innovation in smaller elements of something. It's an incremental process with additional investment along the way, each step going a bit further to proving your team can do what you've set out to do, until eventually you have the whole product.
