People really think a calamity will get replaced? Lol
If they manage to get Kaidou's son to the party, they can challenge a calamity to a fight.
1. You really think anyone of them could beat them? I highly doubt that.
2. When should it happen? The alliance is already on Onigashima and the fight is going to start soon.
3. Even IF a F6 would be able to beat a calamity, their fight would take hours - which they don't have, because the alliance is already there.
And also, even IF a calamity gets beaten, then the one replacing him would be completely worn out, wounded and unable to fight to even close to their normal strength.
It's just doesn't make any sense.