From a purely financial perspective, the answer at this point is no. And there's a very critical reason for this - sporting events are one of the only consistent live TV draws for a mass-market audience today. Both advertisers and media companies have struggled to adjust to on-demand programming, DVR, and the move away from traditional distribution methods. So to find an area where you can still draw millions of live eyeballs to make your pitch without them fast-forwarding through commercials is huge.
Think of eyeballs the way we do home runs. In the 1990s, everyone watched live TV. It was the only option. But now there is a diverse set of options, many of which allow people to skip commercials entirely. Thus, as supply of those live eyeballs goes down, the price advertisers will pay for them goes up. And trust me, every company advertising is cross referencing their lead acquisition costs across multiple ad options so they know what is the most bang for their buck.
At some point, sports will hit a peak due to not enough disposable dollars. But I think we are 10-15 years away from that, and it likely has more to do with general economic trends than just what is happening in sports. The other thing to consider with regard to "bubbles" is that a hallmark of bubbles in many cases is excessive borrowing. We don't really have that in sports. We have businesses that are largely cash-flow positive. And when there are issues, we have work stoppages to sort them out. I don't think there is any sign of a decline in salaries or revenues any time soon.