Of course the potential is huge, but so is the risk of failure. If the chances of success were so likely, why would SD be looking to cash out now?
The wage budget at Sunderland is huge. Cattermole himself outstrips the wage budget of pretty much every league 1 side. Then there are still a number of players of PL, Championship contracts all on several thousand a week. But it's not just the contracts they inherited, you have the likes of Wykes and Grigg who have both been signed on big contracts, and Nelson, Maddison or others will easily double their wages to go there.
They have big crowds, so revenue is also high. But the playoff semi and Wembley crowds have shown that you can't take this for granted. A growing number of their fan base are pissed by the comments of Charlie and the way they see SD as out for a quick profit. Also the novelty of being a huge fish in a small pond soon wears thin. In the conference we partied in Weymouth and took over 3/4 of the crowd at places like Stafford Rangers, but the second and third seasons were a very different experience.
So, buy Sunderland for £20m, spend a further £10m and get promotion - winner. Fail, and you find yourself desperate to sell on!