I'm not sure how much a licensing manager would cost...but I'm not sure there would be much cost at all if the license is simply left hanging.
As far as I know, WOTC has no interest in using the d20 logo, and since 'd20' is no longer part of the 'dress' of 4E, they aren't building its reputation with each new product - in fact, as the market leader, each time they release a product without a "d20" designation, it would seemingly water down the value of the license.
It seems that they could have allowed the license to continue indefinitely, trusting that 4e would be of sufficient quality to drive everyone over to the GSL product. Nota lawyer, but how much 'management' is required for a license that you are leaving unupdated? Could they not also have built a new D20 version, supplanting all prior, that said "all old products may continue, but nothing new"?
I understand WHY they would do it, and I think that from a business only, community-be-durned perspective it might be a shrewd choice. But since they are no longer using the logo/license, it FEELS a lot like a move solely to force people to either spend tons of effort changing old products, or (more likely) causing them to destroy all old products.
(an aside: Let's hope WOTC doesn't present themselves as a 'green' company, based on how much paper they're causing to be sent to the bins)
I think I might have been more okay with it if they had kept the d20 logo, and said that no one else could use it going forward.
In today's economy, I'm moving toward patronizing only small presses anyway, so 4e is out for me...I'll be picking up small house stuff and converting it (when necessary) to True20.