First, you might want to clarify in the first paragraph that you're talking about the existence of nukes, as opposed to the use of nukes. It's clear from reading your entire comment, but I find it's good to play it safe with that sort of topic! ;)
Second, to play devil's advocate, is it possible that landmines could have a net positive impact if they could somehow be properly, globally regulated? For and extreme example, say that only the UN had access to landmines, and they were only deployed as a last resort to enforce DMZs. (Also measures would be taken so that they could be safely removed.) Seems like they could be a positive thing if used correctly. I expect patents are much the same. Certainly overused and abused currently, but not necessarily negative if they could be used 'correctly'.
Kind of like copyleft- but for patents.
For example, in the smartphone industry, where a product is old in a year and obsolete in three, it doesn't make sense to me for patents to last decades. On the other hand, it does seem reasonable to reward significant innovation by granting a patent for a long enough time period to establish a strong market position, before opening things up to competition. The tricky thing is, that ideal time period would be highly variable based on the industry and even the specific idea being patented. Still, surely there's room for improvement there.
Mmm, no, I'm pretty sure that's not possible. It would be possible to have some kind of super-low-casualty 'world war' with fewer deaths than those, but not a knock-down, drag out world war.
On the other hand, to defend patents, you have to show that there are things on the market that we can buy that would not have been made available to us if not for patent rights. Nobody can claim that patents have saved us from a catastrophic world war by forcing our leaders to find less destructive ways to disagree. About the most you can say is that without patents, some lifesaving medications for some relatively uncommon conditions wouldn't have been economically feasible to develop.
Musk's comparison of patents to land mines is much more apt than comparing them to nukes. Only a psychopath (or a land mine manufacturer) would argue that humanity is better off because land mines exist. Nuclear weapons are a genuine gray area in comparison.