So we're back to blaming Disney for the fact that legislators can't be bothered to vote with their conscience?

No, we're acknowledging that legislating is a skilled job, and that experience counts.

Look at the average tenure of a US legislator, look at Copyright Law in the US, and say that again.

Sure: Legislators have to prioritize lots of different things, not just copyright terms, and the skill of making deals while operating within the rules and traditions of a legislature is one they must learn on the job. Having a bunch of short-timers wouldn't make copyright law reform any more likely, and it certainly wouldn't make anyone more likely to vote their conscience. Further, copyright law isn't an issue of conscience for most legislators to begin with, no matter how long they've been in office, as most of them, like most of their constituents, don't understand the value of public domain or the importance of shared culture.