In the service industry, and especially amongst young people, it's common to pick up a job like this for a few months and make some money, then move on to something else. And that's okay, not everyone wants to make a career out of making coffee. It's still a good formative experience to have, while earning some money, and there's no reason why you shouldn't do it right and get rewarded for it.
But profit sharing won't work for someone that is around for a summer. For instance that person could benefit from any upswing caused by previous employees, but if they don't add a positive contribution to the customer experience themselves, they won't be around for the long-term impact. And if you don't give them an incentive at all for the first few months, then you're back to square one. Or even worse: they could negatively impact the profit sharing of employees that have been around for the long run.
Ultimately, money is a motivator and pretending like it's not does not make tipping go away. The problem with tipping as I see it, is that it masquerades the real cost of doing business.