The thing that kills startups at some level, is the founders giving up.
It's become popular in recent years to say that the idea doesn't matter.
I prefer to invest in a company that's going after a small but rapidly growing market than a big but slow growing one.
... you want to be proud of how much you can get done with a small numbers of employees.
Because so few people make an actual long term commitment to what they're building, the ones that do have a huge advantage.
You really want to know your cofounders for a while, ideally years.