I don't think we need a law, we just need businesses to stay greedy.
DRM licenses are kind of expensive and it's hard to publishers to justify the added cost of DRM after the initial release, when they've made 80% of the revenue for the game. So they often (though, not always) get patched out eventually.
When a publisher has made the majority of the expected revenue off a property they're pretty unlikely to want to donate some more dev time out of the goodness of their heart. I think historical trends have also shown that DRM is almost always left in place - so I think the empirical data is pointing strongly at a lack of any organic motivation to de-DRM any products.
Most of the time an anti-DRM patch gets rolled out it's either a company/individual that has strong personal feelings about DRM (i.e. somebody like Stardock) or else it's a patch that the devs wrote way back when to make testing in some environment easier that they kept around and valid out of the goodness of their hearts. I think in almost all cases it boils down to someone who personally disagrees with the prevailing business opinion that DRM is good and actually has enough political power in the company to force their opinion onto the business at large.
But there's a business case to be made for patching out DRM very early in the product lifetime. The longer a product is protected by DRM, the more likely the DRM is to be cracked, and that this crack could be applied to a new releases. It doesn't make sense to put a new game release at risk just to keep protecting some five year old game from piracy.
If you're selling DRM, you want to disincentive publishers for using your protection for too long. And if the DRM is built in-house, you want it to keep functioning for a number of titles to amortize the cost of development.
