I wasn't aware it was possible to hold over rewards for more than one season. This seems like it leaves open a bit of an exploit of its own, though - you could for instance wait until all the other original epics have expired and guarantee that all epics in your season rewards from now until new ones are released would be Manticore.
This is probably not a big deal unless there's something coming that's as dominant as Arianthus, but there will be opportunities to guarantee large chances at each new release card if one wanted to wait long enough.
Yes, I think at some point we will change it so that if you don't claim your rewards before the end of the following season you will lose them, but it's not a high priority yet and we'll be sure to give everyone ample warning before any such changes would go into effect.