The quest won't appear until after you are given the means to kill Alduin. Typical huh?
The Blades are plot aware or something.
Except they do seem to have a tendency to stall out the whole main quest with a interesting variety of bugs and in two different playthroughs they have bugged out at some point requiring them to be skipped,deleted,reset,etc... just to get things moving again.
From a role playing perspective are illogical as they dont care about the Odahviing.
Would you let a serial killer who murdered hundreds of innocent people live if he helped the army assassinate an evil dictator?
Because you believe in redemption, change, etc...?, Saving everyone else + waiting a couple lifetimes and doing it again is pretty much like having a serial killer self imprison themselves and sit doing anything to help except there is no realistic anolog to saving the world twice by stopping a time traveling bad guy.
Also Odahviing is refering to the Way of the Voice as a tyranny which it is from his POV as it implies being peaceful.