There are actually quite a few studies out there that support either side of the discussion.
I've driven to Charlotte twice to see Panthers games, and patronized local businesses, whereas without the Panthers I never would have stepped foot in that boring city.
Opposing fans and non-local fans flood into the city every year to see games. I think these studies that you forgot to link to are wrong or don't exist.
Here is one of the studies that seems somewhat more balanced and makes an intriguing point:
"We should consider the net benefits to the community of alternative uses of the funds spent subsidizing sports facilities. Good policy means using the money where the net benefit is greatest, not simply where the net benefit is positive. That’s something state and local governments should keep in mind before pledging millions of dollars to fund the next new stadium project. And it’s something Congress should remember when evaluating the future of U.S. tax policy."
Regardless, Charlotte is a lot more entertaining on football Sundays now that the Panthers are in town.