Earned rewards should not expire

There are only a finite amount of spaces for heroes, however, during the storyline you will frequently earn more covers. This creates an issue where you are penalized for leveling up your heroes because you will have to sell them to make room for the new hero covers you've earned. This issue can be dealt with, but you have to do it within a specific time period or your new-found cover will expire.
It makes sense to have some rewards expire; perhaps the daily gifts. However, if you've fought your way through difficult missions, it is disheartening to see that your reward (that you can only earn once) will disappear in a week. This would be a bit more acceptable though if you could re-earn those heroes by retrying the same battle. It still makes it suck if you have to ditch the hawkeye that you've only dumped a few levels into, but still represents several battles of hard work.