Yes, solution is underway...

If you weight the benefits vs. the drawbacks with respect to the patent system for each company, where the result is > 0 if there are more benefits than drawbacks and < 0 if more drawbacks, and then you average this value out for each company, will you get a positive or a negative value?

Of course it's negative! But it does not matter one jot. You see: patents are net negative and long copyright extensions are net negative (because they stimulate ever-increased harassment of "copyright violators" by authors who did nothing interesting in last 10-20-30 years and not creation of new interesting works) but they are net positive for the people who can actually pay for congressmen campaigns!

If that's so and if you then add the obvious negative consequences to FOSS developers and users, I think you could have a strong argument that software patents, like legalized slavery, are detrimental to society and should be abolished.

You have an argument, but you have noone to even voice it, let alone fight for it. If you are small startup then usually you don't have money and time to fight patents. Most startups fail (and it'll be true with or without patents), but some survive and grow large and rich. At that stage they have both money and power to fight patents - but why should they? Patents are net positive for large behemots.

Slavery was abolished not because it was "morally right to do", but because there was powerful lobby which wanted to abolish it. Industrial North needed consumers - and slaves are poor consumers. It was "morally wrong" for centuries, but it was only abolished when powerful interests needed to abolish it.

I don't see powerful forces which need to abolish patents today so even if it's net drag on the economy they'll remain and they'll straighten over time.