But you're right, We should have some reasonable knowledge about the odds of success and what to do to maximize them... on the other hand failure is a necessary ingredient of success, and as long as you learn something from each "failure" its just another step forwards.
As for social impact, sure that's great but each individual has their own goals and what matters to them. Some might want to entertain people, or make peoples lives easier, or just create something beautiful. Some want to help the poor or save the world and that's great too.
The one thing I think we can agree on is that its better to work with something that you find intrinsically meaningful rather than being just motivated financially.
Money might be a part of it though, many are the indie developer whose goal is with each project is to earn enough money to keep doing what they love. Or one project might be the stepping stone to making those world saving projects. Last time I checked they sold Wunderlist for 100 million dollars.
As everything you do in life, the startup should be aligned with your values and long term goals in life (aka the purpose you are giving your life), so you can be totally in peace with yourself.
That way, as you mention it, whatever happens financially with the startup, you can be in peace with yourself, knowing you have contributed your part to make this world a better place.
If you do plan to run a startup, and then fail, at least you are prepared for it, and you can still feel good about yourself.
Also, I'd advise to do something that has social impact. If your company is just building the next todo-list, or the next css framework, or the next electronic paperclip, then if you fail to make this a big financial success, you will feel you have wasted part of your life. On the other hand, if you have worked on renewable energy, and failed, then you will still feel good about at least having tried to make the world a better place, rather than just having aimed for the money.