If the Jag's bought the stadium, they might lose all those "hidden" taxpayer subsidies for maintenance, paying rent below a mortgage payment, the City's favorable tax-free bonding, depreciation, etc. More importantly, if the Jags owned the stadium, they would have to pay huge property taxes (10's of millions) that don't get paid now.
Lastly, it would surely make it very difficult to relocate the franchise, even in 30 more years, as the team would have to continually maintain the stadium in first class condition and then be stuck with a white elephant if they moved away.