Disclaimer: I've never bothered with an alliance battle. I figured it was way more apt to draw lots of ego's and thus GM use. That being said:
I have two separate points to make on this topic so I'll make two posts.
If this is really a problem, the real/better solution is to punish players who go AFK at any point. EVER. Many games do this. This game has a problem in that many of the most attractive stats are not "ELO" based but just pure counting stats. There are likely some players who start dozens of maps and quit most of them. They score some points for counting stats on all of them and cherry pick the most advantageous for their efforts.
There are numerous ways Bytro can punish/discourage this behavior. First add a stat for percentage of games completed. It's a lot hard to find allies if you quit half your games and on top of that if I know you're a quitter I'll be much more likely to attack you. This would allow the community to self regulate by adopting behaviors that punish quitters.
Another option is to limit the number of maps players can play simultaneously. This is the case in many games I have played. All the way back to play by mail and play by email games they only allowed players to play on 1-2 maps until they had "finished" a certain number and then gradually increased the "allowed open games" based upon a proven track record of finishing. You could add another game slot for every 3-5 games you finish and subtract another slot for every game you quit. Finish 30 games without quitting and you can play in 12 at a time. Finish 10 but quit 4 and you're only allowed to play 1 at a time.
If there's an incentive to stay, more people will stay. Some play by mail games even stopped charging for turns when you dropped below a certain power level and offered a free turn in a future game for completing the game so that even countries that were being killed had a reason to continue and no vested financial interest in quitting. It was obvious in some games that people quit when they were definitely doomed because who wants to pay $3-5 a turn for the last few weeks while you're already obviously dead.
Does archiving the game make you inactive right away? If that's the case, get rid of that. Don't give people a way to "protect their k/D" by quitting. Alternatively you could saddle people with deaths for every unit that was already created before they went inactive. This would potentially increase the impact to the point players would rather fight (and get some kills) than go afk and take the stat drop.
The accounts here are full of players who have played 100 games and won 2 and only captured 600 provinces in 100 games. There's nothing good about that for the game or the community and way beyond the grindy-ness of finishing the maps there's definitely room for improvement in this arena. The question for Bytro from a business sense is how to make more people finish without impacting the bottom line. I would argue that more people playing the games to the end might actually increase revenue in the long run since the games would seem more fun and more competitive and this might keep more players interested for a longer period of time.