Losing the crypto is easy. You run a shitty company that partners with Circuit City (themselves a shitty company that just went out of business a few year ago), you go bankrupt, you tell everyone that their DRM-locked machines won't be able to play the discs past some cutoff date (despite them paying to have them unlocked forever)...

Then you shut down the servers. Where does the crypto key end up? Who the hell knows. The servers get traded around by whatever company takes possession of the assets, and servers have a short useful life. Probably sent for recycling.

The people who used to work for the company go on to other jobs. Whatever backups they made while working there get put in storage somewhere, to be forgotten, or to be thrown out and buried in a landfill.

And boom! DivX discs (not to be confused with the codec of the same name) are unplayable.

It's probably some half-ass design, and if anyone clever wanted to break it they probably could. But that is of course illegal according to the DMCA, and doesn't really rebut the argument that DRM locks up works that would eventually fall into the public domain otherwise.