Is there any work that is over 50 years old that still brings in big money? The proper solution is to charge an annual fee per work for continued protection of, say $1000/year after 50 years. I'll bet they won't want to pay that.
So what if they're still making money? The idea of copyright term was never to ensure that anybody would profit from them forever. It is to incentivize creators into creating, to help them make some money to making creating worthwhile. Do you think copyright terms extending beyond 50 years would have had any effect on the decisions of people 50 years ago as to whether or not creating something was worth their time? I'm sure Elvis would have thought, "Gee, copyright terms are only 50 years? Never mind, I'm not going to write or perform these songs, sorry guys."