Although I didn't pick up NBA2k15, if it's the same as NBA2k14, then you should be able to play single player offline mode to its fullest. 2k14 servers have been cut off and I'm still playing 2k14 completely offline.
On release week, there were initial complaints about 14 not allowing the gamer to play offline (they're MyGm files weren't working properly; they couldn't boot it up or something). I was massively concerned as well because I'm strictly an offline gamer. Someone said to turn off your internet connection through your PS4 setting (or XB1 setting) BEFORE starting up the game. It worked. I'm happy. Forever offline in my 14' MyGM franchise.
If you're connected to the internet when starting up a franchise, that particular file will forever be linked to the online world. When 2k servers for shuts down, you'll be out of luck. Turning off your internet connection before starting up your game should do the trick. The only concern is roster updates and how that would work. I never updated my 14 rosters. I stuck with the default rosters.