I think you vastly overrate the value of the middle and low end of the ACC FB properties, underrate the Big12 properties, and aren't accurate in saying the ACC's deal is worth far more than the Big12's (ACC schools receive 30m annually). Starting in 2025, the Big12 schools get 50m annually and it contains a pro-rata clause for expansion, with members receiving a full share of the media rights revenue starting in 2025...
You don't see how ESPN can simply decide not to not extend the ACC in 2027? If the top 4-8 schools move on why on earth would ESPN renew in 2027? The schools left would be worth less than half of the current configuration/contract terms. ESPN would at least renegotiate down their annual payout to a remnant league and at most let the whole thing die. So, there is no guaranteed annual share stream in its current state past 2027.
ESPN's option to walk away in 2027 is partially what is causing this all to happen now. The other driver is the expanded CFP and the associated mountain of money that conferences/schools will receive (some are guestimating over a billion dollars annually to be split amongst power conference members). This is a gold rush. And it would make FSU feel more comfortable paying whatever they settle on paying the ACC.
I'm starting to believe that FSU will actually cut checks that are a small fraction of the 500+m. They can negotiate so as to take future monies and pay it to the ACC/schools thereby not being set back huge sums of cash in real-time while being a good threat to be in a 12 team CFP where they would get additional CFP monies for participating.
In this changing landscape, FSU is simply being smart about their future. If the other top 4-8 ACC schools don't do the same, then they're asleep at the switch.