I suppose I didn't make myself clear. Multi-player is not necessarily the only way of "changing your business model" (however, the best solution IMHO). To me, what they need to do is provide services that cannot be "pirated".
For some products, this is support. For others, downloadable content (although it can be argued that this can be pirated as well), general online features (like hosting stats, content, etc).
Stuff like that could seriously improve a customers reason to buy the product instead of pirating it.
What ever you want is going to cost a little more than it is worth. -- The Second Law Of Thermodynamics