If we get some 16 team super conferences SEC could be 2nd or 3rd best conference. Of course SEC could expand too.
Also, SEC hasn't always been king, conferences and divisions go in cycles. SEC will always be good, but sometime they wont be as dominant as they are now.
I would be interested in which combination of expansion of another conference would make a stronger conference than the SEC.
In theory if you added Texas, OU, Notre Dame, and maybe Florida State and added them to another conference together that might make someone competitive at the top end with the SEC but the SEC would still be stronger in the middle to bottom compared to anyone else you added those four to. Also if anyone were to add that combination of schools it would most likely be the SEC. More logical would be two of those to the SEC and two of those to the Big 10, no change in status. The PAC might add Texas, OU but not the other two, SEC is still much stronger.
I acknowledge that relative strength cycles between conferences, in the 70's the Big 8 dominated, at various times the Big 10 or the PAC has been the strongest but consistently the SEC has been at or near the top. Unless something unforseen happens I don't see any change in status for the near future.