If Abiy were to put Debretsion behind bars once Abiy gets strong, that will look like a treachery, a betrayal. Abiy needs to pass a sentence on Debretsion now, while Abiy is still not so powerful. Abiy seems to have confused what he can do with what he needs to do.
Abiy doesn't need to put Debrestion in prison, he only needs to pass a sentence to let the world know that he considers Debretsion to be a criminal.
Abiy can even commute the sentence, but first he needs to find him guilty of all the crimes the TPLF committed
Otherwise people will have suffered in vain.