How future-proof is the arXiv license?
My answer is more to the question How future proof is the arXiv?.
How future proof is anything in this day and age?
The license itself is built on the idea that the system will continue roughly as it currently does, but substantial unforeseen changes both technological and of society (doesn't have to be a war) can easily derail it, as you have outlined in the various scenarios.
However, I don't think there is any reason to worry (your scenarios are too pessimistic), since the arXiv serves a very good purpose to all involved parties:

authors can quickly disseminate their results to all interested,
the scientific community (in fact the world community) get open access to (preprint versions) of many published articles (most in certain fields), and
the journals get a free preprint publication service which boosts their visibility and hence impact (since citations will be to the journal version, not the arXiv version - I know of no journal that disallows publication at arXiv). If a journal reverts this policy (in order to earn on copyrights), most authors of good articles will turn away from it.

Because of these points, funding at Cornell should be possible into foreseeable future.