New blood is usually the most active (most cases I've seen), I don't see a problem with electing a new staff member when another goes inactive, it should be pretty simple to send some kind of message asking inactive staff if they want to step down. Either way, you can't really expect people to stay active to such a voluntary responsibility for a long time, right? Think of it as the circle of life, just it being a lot shorter :3
<-- The actual Minecraft server host