In just a few words:

WINE

Internet Explorer

Active X

Office

Think about it, let us start with Wine here. Releasing W98 into Open Source would inmediately make a lot of code they could use, or at least make hooks for it the customer wants to use. So, by releasing W98 into Open Source, they erased the last barrier to desktop Linux.

Now, IE, Active X, and Office. All these run good in W98 (as well as it is possible for anything to run on one of their OS, anyway). Thus, by releasing it into the Open Source, they would make all that gear instantly available of Open Sourcers. No, don't tell me they could arrange for it lisence wise, as it wouldn be Open Source. If they made it available on a non-Open Source license, open sources would just make plugins for their code which people could just install separatly and get instant compatibility.