Illegal sports gambling in the US is estimated to already be about 1.5 billion a year. If you were inclined to bet on MMA before, you likely already did. Sports betting was a very low priority crime for law enforcement, probably the most interested in going after gamblers were IRS or other agencies looking to get a cut of gambling winnings. So there was so little threat of criminal prosecution that I doubt anyone felt a deterrent to betting. The push to have the law repealed via lawsuit came from state governments trying to create more revenue, so I also wouldnt be surprised to seeing people still using illegal betting over gov't approved just to avoid the stupidly high taxes and fees I expect them to implement.
MMA is a niche sport with a dwindling popularity in the US. I dont think the supreme court ruling that law unconstitutional is really going to provide some huge uptick to Mike's other site. It's even possible that many online gambling websites will find themselves restricted from the US market unless they are willing to kick money back to the US in taxes and fees. That could even lower the amount of mma wagers.