The problem is that you are looking this in terms of only what you can understand in worldly terms. The eucharist does not perish. If you are looking at the terms of the communion wafer then ya it would eventually rot away. But jesus is not talking of the wafer itself but what the wafer becomes. You asked for a scriptural basis for this so here it goes.

49 Your fathers did eat manna in the desert, and are dead. 50 This is the bread which cometh down from heaven; that if any man eat of it, he may not die.

51 I am the living bread which came down from heaven. 52 If any man eat of this bread, he shall live for ever; and the bread that I will give, is my flesh, for the life of the world. 53 The Jews therefore strove among themselves, saying: How can this man give us his flesh to eat? 54 Then Jesus said to them: Amen, amen I say unto you: Except you eat the flesh of the Son of man, and drink his blood, you shall not have life in you. 55 He that eateth my flesh, and drinketh my blood, hath everlasting life: and I will raise him up in the last day.

I highlighted a few important areas. In verse 50 he says that any man that eats it will not die. In verse 52 he then finally reveals what this bread is. HIS FLESH. In verse 53 the jews then acuse jesus of eating his very flesh. In verse 54 what does Jesus do? He repeats himself and expounds by adding "And drink my blood". Jesus continues to repeat himself over and over again. Finally in verse 56 Jesus says for my flesh IS FOOD INDEED and my blood REAL drink.

Yes you are right in that the bread jesus is talking about is a metaphore. But the metaphore is his real flesh and blood not just believing in him. Many of the diciples who heard this left for good after hearing this saying this is a hard teaching. If the bread only meant believing in him then this would be an easy thing for them to understand.

Ignatius of antioch who was a diciple of john had this to say in 110 AD

"I have no taste for corruptible food nor for the pleasures of this life. I desire the bread of God, which is the flesh of Jesus Christ, who was of the seed of David; and for drink I desire his blood, which is love incorruptible" (Letter to the Romans 7:3 [A.D. 110]).

"Take note of those who hold heterodox opinions on the grace of Jesus Christ which has come to us, and see how contrary their opinions are to the mind of God. . . . They abstain from the Eucharist and from prayer because they do not confess that the Eucharist is the flesh of our Savior Jesus Christ, flesh which suffered for our sins and which that Father, in his goodness, raised up again. They who deny the gift of God are perishing in their disputes" (Letter to the Smyrnaeans 6:2–7:1 [A.D. 110]).

Jesus never says that the bread is believing in him. He says that this bread IS HIS FLESH. Why would jesus replace a metaphore for another metaphore?
Catholic have believed this for 2000 years and protestants have denied it for about 300. I'm sorry I can't come to your same conclusion but the language is clear and history is clear.