Just because it'll be there doesn't mean it'll get as much focus as before. CoD and other popular MP shooters still get SP too, but except for maybe Treyarch and 343i, no developer seems to treat SP as more than an obligation. Then you get games like Titanfall or Destiny, which threw SP out of the window altogether or turned it into a weird SP/co-op hybrid.
While I don't ever expect Rockstar to get as bad, how much motivation do you think they'll have for making SP as good as they can, when they'll know most people will buy the game just for Online anyway? How long until the Online protagonist learns to talk and SP experience just merges into Online?