And Enma can't suck haki forever or else that would imply becoming invincible with the passing of time as long as the users' haki is not returned....
I don't see why Oda would portray a sword capable to turn invincible. It can only store haki momentarily, presumably.
For such reason Oden's haki is meant to be gone forever. It's Zoro's haki reminding Kaido of that one.