@johns I'm surprised this isn't the case unless they bury a retroactive grant in the ToS..
@joeyh I think it isn't the case because the ToS say you agree they can be changed at any time. So part of this thought experiment would be that such clauses are not allowed.
The result of the thought experiment could be that ToSes would be even worse, since they would have even more incentive to be as broad as possible at the start.
@joeyh It might matter if you officially canceled your account at that time (there's a separate ToS section for that). But if not, they (and probably others) say that any continued use means agreement to the revised terms: "Customer's continued use of the Service after those 30 days constitutes agreement to those revisions of this Agreement. For any other modifications, your continued use of the Website constitutes agreement to our revisions of these Terms of Service."