Ok I persoanally think that the Phoenix Force and its avatars are always overrated.
What I gathered from reading the comic that was shown earlier is that there are multiple avatars. One in each Universe. The PF cannot act on its own but empowers one person in each reality / universe to take care of everything there. Similar to fixing any little things that start messsing with the Universe.
The Living Tribunal has been around since the beginning of all of the multiverses. He only intervenes when one universe starts amassing more cosmic or mystical power than the other universes. He is there to remain order. LT only acts on things that will ultimately affect the entire multiverse.
LT deals with things on a much bigger scale than the PF. The PF cannot act on its own thats why it needs the avatars. They both have their roles in the Universe though and if one ceased to exist nothing good will come of it.
I will say this though... if the LT is hit by a Shi'ar spaceship blaster... I am pretty sure he would not die... like Phoenix did. So thats my look on it.