> If that asset generates revenue for 120 years, then it’s slightly more valuable than an asset that generates revenue for 119 years, and considerably more valuable than an asset that generates revenue for 20 years.

Not so, because of net present value.

The return from investing in normal stocks is ~10%/year, which is to say ~670% over 20 years, because of compounding interest. Another way of saying this is that $1 in 20 years is worth ~$0.15 today. A dollar in 30 years is worth ~$0.05 today. A dollar in 40 years is worth ~$0.02 today. As a result, if a thing generates the same number of dollars every year, the net present value of the first 20 years is significantly more than the net present value of all the years from 20-120 combined, because money now or soon from now is worth so much more than money a long time from now. And that's assuming the revenue generated would be the same every year forever, when in practice it declines over time.

The reason corporations lobby for copyright term extensions isn't that they care one bit about extended terms for new works. It's because they don't want the works from decades ago to enter the public domain now, and they're lobbying to make the terms longer retroactively. But all of those works were already created and the original terms were sufficient incentive to cause them to be.