> Can you think of any medical treatments that were developed without the promise of 20 years of limitless profits? I can.

This sounds like it's responding to a strawman. Proponents of the patent system aren't claiming that zero innovation would happen without patents, just that it would be much less. In other words:

No patent system: only inventions from altruistic inventors

Patent system: inventions from altruistic inventors and profit-focused inventors.

Let's say corporations were only promised a 10 year monopoly instead of 20. Do you think their business would collapse, or would they keep trying? If profit-focused inventors could still be motivated by a 10 year horizon, then that extra 10 years only benefits corporate executives and kills patients. So, is the patent system as limited as possible while still encouraging innovation? Obviously not, because pharma companies routinely direct billions of dollars in profit into stock buybacks instead of research or any other actual investment. They can easily afford to make less profit.

>Do you think their business would collapse,

It would depend on the project's margins and/or sunk costs. A project that had very little invested in it or is a breakout success might we able to weather a 50% cut in exclusivity period, but a marginal product might not. Most of the already developed drugs are probably going to be fine, assuming that the financing for them was already secured.

>or would they keep trying?

For the projects at the margins? most certainly. That is not to say all private drug development would, stop. If some sort of unpatented-but-super-cheap-to-perfect-but-still-not-patented drug showed up it might still be developed, but in aggregate I'd expect drug development to drop significantly.

I am arguing that there are no marginal projects at current profit levels, and that's why they can spend billions on stock buybacks. There is a point that lessening patent protections would cause companies to cut back, but we are not at that point.

>I am arguing that there are no marginal projects at current profit levels, and that's why they can spend billions on stock buybacks

How does the act of buybacks imply that there are no marginal projects?