You can get a lot of NFL and college football from the antenna. I use my parent's cable login to cover the rest (WatchESPN) but of course that's not a real solution available to everyone.
NFL is doable although there's no obvious solution for MNF except using someone else's login.
NCAAF is a little random since some games are on non-cable but a majority aren't. I'm on the west coast and the games are usually on FS1 or Pac-12 network.
NBA is pretty much impossible until the ABC has their Sunday games and the NBA finals.
Pretty much all of the MLB stuff (except for playoffs) is cable only. NFL will at least of the either Monday or Thursday game along with your local on Sunday.
OTA FOX only gets one or two playoffs games and the World Series.
In previous years it was possible to watch most of the games online via "live alternative video feeds (excluding the broadcast feed)", and now even that is not allowed.
I suspect that some league (NHL perhaps?) will specifically exclude webcasting from its next huge TV contract. Or it will extract a higher price not to be the first to do so. It will be the beginning of the end for regional sports channels, and eventually for ESPN.
For the cost of one to two months of Cable TV you can get all the regular season games online; since I'm a fan of an out-of-market team anyway, it's a no brainer for me.
Unless you want to watch a lot of sports.