If they start losing copyright, they start losing back catalog. I doubt there's a huge demand for the 1920s Steamboat Willie [1], but in 2032 they'll lose Snow White and the Seven Dwarves and in 2035, Pinnochio.
Of course, that's an awfully long ways off by a time-value-of-money analysis at the rates a modern company would normally use, so the other thing to look at and be... outraged? perturbed? surprised?... at is just how goddamned cheap Congress has historically been. Did you notice this line:
"In one instance, Eisner paid Senate Majority Leader Trent Lott (R-MS) $1,000 on the very same day that he signed on as a co-sponsor."
I mean, I presume that's not necessarily their only cough "donation" to the Senate Majority Leader, but one presumes that the Majority Leader ought to command a price premium, too, and for $1000 to be a notable donation to talk about, well, holy cow. I'd bet Disney wouldn't pay $10 billion as a lump sum in 2023 to extend copyright again... but we're not talking anywhere near that sort of money to get copyright extended. If they can do it for <$10 million, the cost/benefit is a no-brainer.
Remember, the value to Disney only has to exceed their expected costs. If their expected costs for a copyright extension is an accounting rounding error, of course they're going to try to extend it.
[1]: Which, if you've never seen it before, I recommend giving it a view: https://www.youtube.com/watch?v=BBgghnQF6E4 The past is a foreign country!