If you're a small team with limited runway, you can't simply decide to disrupt an entire industry. That's a huge undertaking, and a hopelessly vague one. What you can do is bite off a small piece of the problem, find some customers who really want to give you money, and iterate. Once you have a proven track record and solid revenues, you can aim higher.

Once upon a time, VCs didn't insist on taking baby steps, and would fund people who wanted to "revolutionize the grocery industry." That led to disasters like Webvan, and the first dotcom crash. For a good history of how Webvan bit off more than it could chew, see Four Steps to the Epiphany. For some online overviews, see:

http://articles.sfgate.com/2001-07-12/technology/17606161_1_...

http://www.beyondvc.com/2010/10/dont-build-an-empire-overnig...

Today, you're still allowed to dream big. But you're expected to start small, and build from there.