I think the interesting part of the sale is not that Bethesda bought the IP, but the MMO deal.
Keeping in mind I breifly read through the responses and haven't clicked a link yet, on Beths end, if they weren't interested in making an mmo, or haven't recieved much interest in people liscensing an mmo on FO from them (and who would? What dev studio willing to acrue the debt needed to make an mmo would want to acrue more debt using this IP instead of a new one without restraints?)
The only logical choice Bethedsa could make if Interplay was willing to lower the rpicetag in return for having an mmo-deal would be to make the mmo deal, if that saved them enough to offset the opportunity cost of being able to liscense that out to someone else for more. If the opportunity wasn't very likely or as profitable as the actual gain of a lesser pricetag on the IP, its a no-brainer to take the better deal.
But Wow is a cash cow, so Bethesda must have plans to steal some of the economic profit blizz is making. But what IP would be a stronger one for them, FO or ES, for an MMO? Probably ES. It seems far less risky (in my opinion of course). And if Interplay actually pulls off the mmo, Beth can make some nice profit with no work at all.
By why would Interplay give up real cash for an mmo deal? We know he had plans to make an mmo. But anyone willing to give interplay enough debt to make an mmo is going to have a return worked in that fits the risk. So with Bethesda taking creames, and the debt issuer taking more creames, Interplay would not be making that much real profit on a succesful game if made. But if not made, Interplay is in the same boat as before.
More cash still leaves them in debt or without enough working capital to do anything still. Debt is a given. If they lose the IP back to Bethesda they are still just as insolvent as if they never had it. The mmo deal is just a chance for them to get on track. The chance might be ever so slim, its still better than no chance. Even if they make nothing off of the profit of a succesful launch, it the profit after expenses and op costs and before paying debt and Bethesda look good enough, they could get better debt to make another non-FO game, then another, then another. And be back in business again. Thats how I see it anyway.