> In that case, they’d just shred the masters unless someone offered to buy them for enough — and for most stuff, no-one would.
Unlikely. Shredding costs time and money that no one will allocate.
Suddenly you have: "Well, these things are about to come off copyright, so if we don't get money now, we're never going to get any at all." So, someone looking to add a few bucks to his bottom line budget in the company will almost certainly toss them out at a reasonable price.
Contrast to now where the risk is that you sell off "Random Garbage 42" for $10,000 that happens to have a 10 second cameo of Elvis in it, and it becomes worth $250,000.
In that case, they’d just shred the masters unless someone offered to buy them for enough — and for most stuff, no-one would.