I don't know what will happen in the future. However, if the SEC wants to separate, they will need more teams. They have teams in the Southeast and the Southwest. Will a league made up of only SE and SW teams appeal to the entire country? They will need teams from the NE, Midwest, Northwest, Mid-Atlantic, West, and other parts of the Southwest. The SEC would have to add some of those regions before attempting to separate.
If they attempt to form their own league with 16, or 20, or 24 teams mostly from the Southern USA, the rest of the NCAA might just decide to not allow games against the new league and keep chugging along. Is a football fan in Los Angeles going to watch USC vs Oregon or Alabama vs Florida? Will there be as much excitement about an 8 out of 24 team SEC playoff when they have all played a lot of interconnecting games during the season? MLB intraleague play is no interesting any more. There is no novelty in watching the All-Star Game or the World Series. Currently, there is argument during the season about whether teams like Ohio State or Oklahoma or Alabama are better. The CFP is interesting in part because it puts together teams from different regions who play different styles against different competition. No matter what people in the Southeast think about the SEC, a league of mainly SE teams with a few SW teams will not be very appealing to the rest of the country.
I am not saying that a group can't split from the NCAA and be successful. I am not saying that the SEC cannot be the group to do it. What I am saying is that I don't believe SEC currently cannot accomplish that. They will need more teams to accomplish it, and not solely based on football success. (They would need a presence in the NE, and there aren't any football powerhouses there.)