I'm sorry, but that didn't make sense to me. The point of copyright is to allow the creator control so as to make a living. That's the further encouragement. First time you starve, everyone understands that. Subsequent iterations should get progressively easier if your work is desirable.
That's not true. The point of granting exclusive rights through copyrights and patents is to encourage the creation of works which contribute to the overall good of society. We don't care about individual creators past the need to encourage their creations, and further, due to the transferability of intellectual property, any post-creation changes to copyright law would not only provide no additional incentive for creation, but would benefit copyright OWNERS, not copyrighted material's CREATORS (though I concede significant overlap).
Think of it like this: intellectual property is a bonus you receive at the creation of a good which is very cheap to copy. Changing the value of this bonus UP, retroactively, costs society for no societal goal. Changing it down is similar to breach of contract. There are strong arguments for fixing the length of the term at the time of copyright.