The cannibalsim argument proceeds from a misunderstanding of what substance is. What a cannibal eats is the accidents of a body, not its substance.
A cannibal eats the flesh of its victim. That IS its substance.
When we devour food we eat all of it. Both what it truly is - its substance - and also its ephemeral accidents.
The Eucharist is not an act of cannibalism - not because of accidents vs substance, but because We eat Christ.
He is not dead flesh, but uttermost, living reality. He is the true Passover Lamb.
And - like the Jews in Egypt we eat Him to save ourselves from death. We eat of our Saviour, the true Lamb of God who is prefigured in the Passover meal.