I find this to be one of the most annoying features that can't be controlled in Trello. For example, we manage a list within which we have an "idea dump" of cards. Lots of them. Those cards are not scheduled, used or whatever for long periods and a bunch of them appear greyed out eventually (makes them look 'inactive').
I wish there was 1) a preference for this NOT to happen at all, or 2) a menu item (function) on the 'list actions' menu where I could say "reactivate all cards in this list" or something to that effect.