User blog comment:FairyRaven/Fairy Tail Chapter 465: 400 Years, Review and Commentary/@comment-1359322-20151207213203/@comment-1359322-20151207220532

Internal conflict. Alvarez function like an army like organization, their members aren't fully on the same page, you can see that from the past chapters, if Zeref dies, there will definitely be an internal struggle between themselves. Plus right now it's Zeref + Spriggan + Army, remove Zeref and you can make the army lose their moral as well. At least you can be sure that the East side will lose their morale.

Fairy Tail still has Fairy Heart, just like Alvarez is trying to use that to defeat Acnologia, FT also has that option.