How does one change the world with a PhD? One's contribution is very little among all the other hundreds of researchers that are trying to work out that one problem and most of the time, your PhD doesn't have an impact at all.
I came across a joke once that resonated since it felt like truth being spoken in the clothes of humour -- "When you finish a bachelor's degree, you think you know something about something. When you finish your Masters, you realize you don't know anything about anything and are in awe of other minds around you. When you finish your PhD, you realize that nobody really knows anything about anything, including you."
The offshoot of that last realization is true humility that shows as a deep appreciation for the steady progress being made at the boundary of knowledge, and the wisdom in finding the right question to ask.
To put it another way, both endeavours have tremendous impact on the person. If "finding out about stuff" is your thing, getting a PhD would be worth it. If "doing stuff" is your thing, a startup would be worth it.
PS: This one just submitted his thesis and is considering starting a company.