so there is a need to incentivize research and development.
If you start with this (incorrect) position, then of course patents seem necessary.
Who would invest hundreds of millions of dollars in research...
The simplest answer is that you don't need to spend hundreds of millions of dollars if you do incremental improvement -- which is only possible if there are no patent protections. The patent system itself makes itself appear necessary. The simplest example of this is the inherent differences between how Linux (and most open source) is developed in contrast with MS operating system. By incrementally making small changes and releasing often Linux as surpassed Windows in terms of quality etc.
...could copy their invention and sell it for the marginal cost of production
There is a cost to copying which may not be as high as the original, but it is non-zero. Also, if you are the one innovating you will have the lead in market; during this time you must continue to innovate to maintain your lead. Your continued innovation will leave you at the front while others lag. But that's a lot more work than relying on government monopolies.
but a substantial number of these products would not exist.
Pure conjecture. But I will grant you that some products we have today might not exist in a non-patent world if you agree that in our patent-world there are products we could have and don't because of patent issues. Whether we are better or worse of as a result is purely imaginary. I suggest we are worse off as necessity is the mother of invention; so any product with a need that can't be met today due to patent is a loss to us.
While I have not read the original posts you reference, WTO, FTC, US Courts, EuroCommish, and EuroCourts are not agencies I would trust for original research.
So I see your Schumpeter and raise you a Boldrin & Levine; the research and studies they quote and use seem to strongly indicate that there is no gain by giving monopoly protection. And that's actual in numbers, not hypothesis or theoretical discussions.