I've spent a lot of time following college football conference realignment over the years, which meant paying attention to things like attendance, endowments, athletic department revenues, state population, TV ratings, etc...
The SEC has no real path forward to leave the NCAA by themselves. That's important to understand. They have two really good targets to expand their territory, and then things fade fast to the point of losing money for each addition. There is no geographical power grab for the taking with the SEC.
This means that you need a partner to leave the NCAA, otherwise you're turning yourself into a far less relevant regional player. The Big 10 actually has far more expansion targets if they wanted to go at it alone, but they would quickly be faced with the same reality of losing money for each addition they make to cover more of the map.
Even if the SEC and Big 10 were to leave together, they create another problem where there's a giant chunk of the country and a lot of nationally relevant brands left out. This means they can't even have a 32 team basketball playoff, they can't have a 16 team college football playoff, they're not in an ideal situation. There's basically 4-6 teams that are good targets for the SEC and Big 10 combined, and that just won't cut it.
The only realistic path towards a breakaway actually involved the Big 12. The Big 12 holds important territory that neither the SEC nor the Big 10 want (for instance the block of Kansas, Arizona, Colorado and Utah). They could also absorb the remaining relevant ACC schools neither conference wanted (like Louisville, Duke, etc...). This is also relevant when you consider this break-away would still need some sort of legal cover/congressional action, without having most of the country on board they'd never get beneficial legislation passed nationally.
The reason this would work is TV deals. Yes, the SEC and Big 10 would have to share post-season revenue with the Big 12 in this scenario, but their TV deals are not diluted by adding a lot of teams that in fact cost schools money to add. So, basically this is the only way I can see that the Big 10 and the SEC actually break away from the NCAA but also gain money instead of lose money.