Agreed the primary objective is solutions to problems. I tend to think the best solutions will very often come from entrepreneurs and startups but that need not always be the case.
However, on the margin, you should choose to solve the biggest problems for the most number of people. If you succeed in doing that you may be able to build a venture scale company (and arguably you should). Furthermore, it's hard to build a company either way, so you might as well try to solve the most ambitious problems you can.
On the Square point - I don't know Jack Dorsey but I would find it hard to believe that he chose the "street vendor" problem as it was the first thing worthy problem he saw after Twitter. Furthermore just the problem statement is not enough, making X easier is not an idea. *Ideation is was happens after you've noticed that initial problem.* A nuance that seems to be lost when people talk about startup ideas.
