First sale doctrine doesn't obligate Ubisoft to honor the key.
Exactly right! What a lot of people don't understand is that the First Sale Doctrine is a defense not an offense. In other words, if you buy a copyrighted item, like a book, and resell it, the First Sale Doctrine protects you from getting successfully sued by the copyright holder for doing so. In other words, it is a defense. It does not however, put any obligations on the publisher to provide any support to ensure that these later customers can use the product. I'm not saying Ubisoft is doing the right thing here, but this really has nothing to do with the First Sale Doctrine.