Florida State and Florida play each other every year. They are also a good distance apart. Florida State to the SEC should not effect Florida at all.
The SEC is a money-making machine. Each member school wants to retain exclusivity within their home state to avoid market saturation and redundancy.
In the case of preexisting rivalries like Duke-UNC, the in-state competition provides mutual benefits.
The difference is Florida would continue to excel as the sole SEC team in their state. If FSU joined, they would encroach on Florida.
If Duke or UNC defected to another conference, both teams would suffer since "success" in their eyes are measured against each other.