I'd be willing to bet that some collusion has already taken place. They must already have an agreement. Microsoft probably was the one that really wanted it, and Sony was willing to agree, as long as Microsoft got more out in front of the issue, and then Sony can look like they are just hopping along for the ride. Sony probably wasn't as insistant as Microsoft about this strategy, so they'd prefer that MS takes a bit more of the net rage than they will. MS is in a stronger position financially, so they can look a little bit more like the bad guys, and it's not necessarily going to kill their company. Sony is on dangerous ground, so I don't see them championing this anywhere near as much. I expect them to eventually follow suit before launch, with their plan to have the disc marry the console.
Both Microsoft and Sony could "allow" a one time license transfer for a fee of $15. Your rights to the game would be revoked, it would delete off your system, and would transfer to another system. It can only be done once, and there is a $15 fee paid by the buyer to activate the disk.
I also would expect that both Sony and Microsoft would offer a rental plan of some kind. If games are locked to one system, then there wouldn't be a rental market either. I would expect both companies to fill the void digitally. You can rent games and download them level by level. The fees will probably be similar to RedBox / Blockbuster.
Basically, I see both companies trying to kill several industries that are leaching off their products. You have the used game market, the rental market, and possibly the pirate market (if their always online system has the right security). They want to replace two of those, with their replacement version, except with them getting all the money. Think about every bit of revenue that GameFly and Blockbuster and RedBox and all the other rental places get from renting games. All that revenue would go directly to Microsoft and Sony (to be shared with the publishers of course). Same thing with the used game market. Used games can still happen, it's just that any time a game changes hands, Microsoft and Sony get $15 (to be shared with the publishers). Also, an exchange can only happen once. One game doesn't end up constantly being flipped to the next owner. It's a one time deal.
These companies have to do this now, because we could be looking at another long gaming cycle, possibly more than 5 or 6 years. I'm sure MS and Sony might not want to rush into the no used games thing right now in 2013, but at the same time, they know that these new systems could have to hold their forts down for the next 7 or 8 years. They have to think long term. I'd be dollars to donuts that they will both have DD only versions of their respective systems before their lifetimes are over with. They know they can't get away with it in 2013, but in 2018, it could be a totally different story.