I don't know how you came with that conclusion... But this FF isn't that heavy.
Because Athena has more utility alive than dead. As simple as that. Medusa is not Yoruichi that Kisuke would rampage and massacre for. If someone is of use to him, he'll use him or her, even if they are his enemies. That's the reason why he was able to cooperate with the Arrancars.
One requirement to become a Captain is to have a bankai. The only shinigami who broke that rule is Zaraki.
"Are you trying to say that Byakko could help me? But I can't even hear him most of the time and he never listened to anything I said... It's already a miracle that I reached Bankai form with him..."
Anime & Comics · Goyya
Kisuke should reappear at the end of 523 if I recalled it correctly.
Playing with other Supernaturals
Anime & Comics · Goyya