Geocaching (formerly Groundspeak) has decided that it is possible to assign and remove favorite points to geocaches at any time for any reason. From
When a geocache is archived the Favorite Point remains with the geocache. Removing a Favorite Point from an archived geocache and awarding it to a new one is up to you. Visit your Favorites List and look for geocaches with the red strikethrough indicating that they are archived, and then remove the geocache from your Favorites List to free up the Favorite Point. There is nothing wrong with leaving a Favorite Point on an archived geocache if you prefer it.
I would personally never retract a favorite point that I assigned on purpose. To me, that point is earned by the cache and its owner. Even if we ignore the question of belts and badges it gives an impression on a given geocacher's ability to make quality, worthwhile caches. If favorite points are removed from that CO's caches, information that could be used to judge who is generally good at making worthwhile caches and who aren't.
Part of the exercise is to pick the one cache in ten that is slightly better than the others. The experience you had finding it did not deteriorate when it became archived. If you really live in an area where you feel that all the caches you find earn a favorite point, you are blessed with good cache owners and should not complain. The solution to not having enough favorite points is IMO to go find some more caches .