I understand the frustration in the case when someone's account just goes dark but not if its just "well they're not leveling it yet".
In the case described here, you're going to get the reward you have earned eventually. It may not be on your preferred schedule, but those FP are still an investment.
In the case of players who leave th game, new donors are not going to level the GB, so you're forced to either invest WAY more to level it yourself, or try and get others to kick in with you, or just let those FP die. It is frustrating, but there's no good solution to that.