Thanks for the email. In answer to your objections:
i) I don't need to defend my own interpretation to show that the opposing interpretation is flawed. I only have to defend my own interpretation if I advance my interpretation as an alternative. I have no problem with your question, but we need to be clear on the burden of proof.
ii) The first question we need to ask in reading the Bible is not, "What makes sense to me?" but, "What would make sense to the original audience?" given its cultural preunderstanding and position in redemptive history and progressive revelation.
Jesus was addressing a Jewish audience prior to the institution of the Lord's Supper. The Eucharist was not their frame of reference. In context, their frame of reference was the wilderness wandering and the feeding of the 5000, which immediately precedes this pericope. They were the target audience, not us--and their historical horizon supplies our own compass points.
iii) Bread is the staff of life. Manna fed the Israelites for 40 years in the wilderness. Hence, it prefigures the sacrificial death Christ inasmuch as the type and antitype alike are life-giving, but with a difference. The shed blood of Christ confers eternal life upon his people.
iv) A metaphorical reading of Jn 6 makes no less sense than a metaphorical reading of Jn 10 or a metaphorical reading of Jn 15.
v) Meaning is a relation. What we mean is how we mean our words to be taken by our audience.
Ordinarily, you write and speak to be understood, which takes the common knowledge of your reader or listener into account. There's a lot you don't say and you don't need to say because language is a social code with a background of shared assumptions.
I don't think that Jesus went out of his way to be unintelligible, do you?
vi) Sure, Jesus can teach something knew. But telling an audience something they can't possibly understand is a pretty poor teaching technique. They can't learn what they can't understand. Surely Jesus was, among other things, a master communicator.
vii) The question is not whether your audience can misunderstand your words, but whether they can understand your words.
viii) The problem with the Pharisees was not their rejection of new teaching, but their rejection of old teaching. Jesus constantly reasons with them from the OT.
In some respects, the Pharisees had a pretty good idea of what Jesus was getting at. They saw him, not without reason, as a threat to their hegemony. He repudiated the authority of the oral Torah. And if Messiah has come, who needs the Temple anymore?
That's one of the ironies threading through the Gospels. His enemies are often quicker to pick up on his claims to divinity than are his followers.
ix) Likewise, the disciples are constantly reproved for their failure to grasp what they were in a position to grasp.
x) There are many specific objections to the sacramental reading of Jn 6. The hermeneutical objection is of broader importance because a flawed hermeneutic will introduce a systematic error into our reading of Scripture generally. Among more specific objections:
a) If the sacramental reading were true, then every communicant is heaven-bound without exception (Jn 6:54). Do you believe that? Do you believe that everyone who ever went to the communion rail is saved?
That's not catholic theology. In catholic theology, sacramental grace is resistible. And there are various impediments to the right reception of the Eucharist, viz., an invalid sacrament, a wrong intention on the part of the priest or communicant, the wrong communion elements, &c.
b) There's an interplay of literal and figurative language in Jn 6. Literal faith in Christ (v47) is picturesquely redescribed as eating Christ (v50f.).
c) John is fond of spiritual metaphors. Why take Jn 6 literally, but Jn 10 or Jn 15 figuratively?
d) Like the synoptic Gospels, the Fourth Gospel is basically self-contained in the sense that the Apostle John can't assume that his readers have access to Luke or 1 Corinthians or other NT books. So his work needs to be comprehensible on its own terms.
Now, the Fourth Gospel doesn't record the institution of the Lord's Supper. So, again, his readers don't have that point of reference when it comes to Jn 6.
e) Although I doubt you're a big fan of Calvinism, there is, nonetheless, a deep predestinarian strain running through the Fourth Gospel. Indeed, it is on display in Jn 6 and elsewhere.
But sovereign grace is at odds with sacramental grace. If you channel saving grace through the sacraments, then it can't be sovereign since sacramental grace is indiscriminate and resistible whereas sovereign grace is discriminating and irresistible.
f) A leading theme of the Fourth Gospel is the culpability of those who refuse to take Jesus at his word. But if his words in Jn 6 are simply incomprehensible, then in what sense are they guilty of unbelief?
g) "Flesh" is an allusion to the Incarnation (1:14), not communion.
h) As I point out in my recent essay "On taking John 6 literally," the sacramentalist backs away from a literal reading of Jn 6 by introducing distancing devices to insulate his claim from palpable falsification.
i) On a more general note, there's a tension between faith in Christ and sacramentalism. Those who view the sacraments as a means of grace are trusting in the sacraments, the priest, the church, for the source of salvation, and not in Christ. The sacraments become a surrogate Christ--a substitute for the real thing.