> Opt-in would hugely limit the amount of material - possibly to the point where AI research becomes unviable for many applications.
Not necessarily a bad thing. If your tools require widespread breaking of existing laws, your tools are broken.
> it's a legal trade-off that should carefully weigh up cost vs benefit.
Cost: Loss of millions of jobs which are suddenly invalidated by the loss of copyright. Less content overall being created.
Benefit: AI can freely consume what content is left.
I think the cost is far too high.
EDIT: I'd also point out that the "natural" state of man is no laws at all - ownership of goods is enforced with only your strength of arms. I really don't want to live in that kind of world.