The problem is that they know all their own libraries still make them money. People are still making from the White Album.
As that anchor is dragged forward, those artists and albums at the back stop making money for them. And then they realise that as that anchor is inexorably drawn forward from that point on, they lose more money every year because it's likely the new artists aren't making as much as the old back catalogues (maybe individual examples, but not overall).
And then they realise that, in 50 years time, when all they have to monetise is the junk that they've been churning out recently, they are dead in the water and the industry will struggle to sustain itself. They're not saving themselves for today, but for their retirement, when they're basing their business on people buying Britney Spears' back catalogue etc.
That said, any law that has to be revised the number of times that the copyright ones have should really be scrapped or made indefinite. If NOBODY in a certain industry (music industry, Disney, etc.) has ever seen their copyright expire, how on earth can we say that we need to legislate to extend that protection continually - and multiple times - without making the case that it should be indefinite or not?
I'm not saying that's a GOOD solution, but someone needs to review the time and money spent messing about extending laws to cover timeframes - including overruling laws retroactively - and either fix a date in stone or make it indefinite. Pretending that it will eventually end up in the public domain while that never being legally possible is just outright scumbaggery.