Substitution was the first idea that hit my mind when I faced that problem for the first time.
But in the end, I'm not in favor of this idea, because this game involves a very strong social aspect, and such feature would alter it.
Leaders have to manage and care about who they recruit, who they include in wars, and the best clans are the ones who have friendly, loyal, decent players. But in the first place it's all about how the leader manages that. And it's an entire part of this game.
If you've recently started your clan, you will have new guys who aren't loyal yet, and you can't blame them for leaving, yes it sucks but it's up to the leaders to guess whether they'll stay or not. If your clan is not that new but you still regularly have leavers, that means you're including too many new members at once in wars, or that your current members are still not loyal enough. It's all up to the leader to manage that.
Also, each member is given two attacks per war, and one of the reasons for this is to compensate leavers. Now if the remaining members still can't get enough stars to win the war, that means your members are just bad. Again, it's up to the leader to manage that, you'll eventually have to teach them strategies, or kick them.
In my clan, we're quite decent and we always get at least 95% of the stars, without using all our attacks. That leaves room for a couple new recruits every war. But remember, the smooth running of a clan initially depends on the leader and coleaders. If you can't get your clan running fine, you're still missing something to be a leader (no offence, maybe it'll come
http://forum.supercell.net/images/smilies/wink.png)
By letting people hop in wars to substitute leavers, that will make the managing too easy, and being leader is an extremely demanding role. By keeping that role difficult, it encourages most people to stay in clans rather than have everyone willing to start their own clan to be a leader.
Btw that is also a reason why Supercell added Clan Levels, IMO. If someone is in a good clan, it will discourage them from starting from the scratch just for the sake of being leader.