They can, but that would ruin the success of Skype, reduce its user base and therefore be a loss for those who bought it.

You see, the bad thing about Skype is not so much codecs used... It's the mafia-like "closed network" method; the same used by Facebook or Twitter, btw.

The reason why Skype has such a massive user bas is that you cannot communicate with a Skype user without being on Skype yourself. The network's closed. No open protocols, standards, interoperability. So it grows and continues to grow because whenever one of two lovers is on Skype, the other one needs to join. And the more Skype users there are, the more "gravity" it gains... If 9 out of 10 friends use Skype, why wouldn't you?

Now sure a potential buyer could put an end to this, open up the whole network, make the protocols available, document them as RCFs, allow other services to use the Skype protocol for both clients and servers... What would happen? Skype users would switch to services that do one or two things differently than Skype, but are interoperable. They can still phone all their contacts on the (now open) Skype network, but don't have a contract with Skype (the company) any longer.

This would of course be a good thing for the users and the internet. But it would ruin the only value Skype has for a potential buyer: its user base.

So yes, Nokia could open up everything after they bought Skype. But that would be like buying an expensive car and replacing the engine with flower pots in order to make it a nonpolluting object. It's a theoretical possibility but it's not going to happen.