It won't be multiplayer only. That's ridiculous. Every game that has done that has underperformed commercially and gotten bad press for the decision. Titanfall and Battlefront being two huge recent examples. SWTOR's most celebrated parts are the portions of the game that can be played entirely be yourself. The best reviewed and most celebrated games are single player games first, and if they have multiplayer it's not the focus. Moreover, it's been difficult for console based shooters to get acclaim without single player campaigns (PC shooters like Counterstrike and Over watch are a different beast). If someone names the best games of the last 15 years, the majority will be single player. Sorry, it's not going anywhere. There is a ton of money to be made off multiplayer, but games that ignore single player don't tend to be those games.
Now, I do think GTA Online is why expansions were axed for GTA V, which sucks, but if GTA V started out as an online only game, it wouldn't have made the kind of money it did.