I agree it’s a good change but you have to think why now. It’s because nobody is waging wars anymore after the rebalancing. When was the last time you got a notification to take revenge for an attack? I get it now. This is what they have come down to.
I think we can just wait to see how it would roll.
I assume that the check will be made right when this new rule starts. So alliance that hasn't waged wars H-15 new rules apply will be rolled back to 12k glory. As well as those alliances who have fewer than 10 members.
For example : if those KA and Samurai alliances in top 5 leaderboard want to survive they must be refilled again, and/or wage wars 15 days before this alliance decay rules start to apply, otherwise they will drop to rank... 100k?! Then they will have to wage wars again for the next 15 days cycle. I like this idea! I'm not a fan of leaderboard monopoly. Seen this monopoly in several other games and it just made my head warm.
Other option, when they announce the rules, alliances will be given 15 days cycle to wage wars and/or collect members or their previous glory will reset to 12k.
I haven't read carefully the post to study it. But I guess both options are possible.