I think five year increments might be a little short, but I agree with the general idea. One of the biggest problems with indefinite copyrights is that you wind up with big chunks of culture that get sucked up into this copyright black hole. They are too locked up to safely use, but it's difficult or impossible to tell why might actually own the rights. Then it's only a matter of time before large sections of our cultural past fade away forever.

"At the 150 year mark Disney can decide for themselves whether another 5 years is worth a billion dollars."

That's one reason that the plan would fail. One of the reasons I like explicit registration is that allowing these sorts of indefinite extensions for some works makes it less likely that companies would lobby heavily against the plans. By making it deliberatively punitive for these scenarios, you put the plan up against the same forces that are already fighting (and successfully, at that) for blanket copyright extensions.