Something to add: Nibbana is liberation
If it were extinction, that would not be freedom because action would no longer be possible. If it were something like birth in a heavenly realm above the highest heavens of samsara, it would not be freedom because action would be required; you would have to be a sentient, acting being, subject to the laws that govern that realm.
With total liberation, action is possible but not required. It is liberation without a self to be liberated, neither action nor non-action (action and non-action do not apply). The distinction between annihilation and eternalism seems to develop from self-thinking, thinking in terms of gain and loss.
The best things in life aren't things.