In John 6, Jesus is already speaking figuratively by comparing himself to manna from heaven.
The problem with the point you try to make is that the Jews were grumbling about more than being told to eat flesh and drink blood.
So the “hard teaching” wasn’t solely concerning eating flesh and drinking blood. That wasn’t even the first point made that led to all the grumbling.
Very truly I tell you, you are right that those verses lead to the path through the gates of heaven. It’s just that they don’t say anything to prove transubstantiation as true.
Of course he was speaking figuratively when he said,
“I am the bread that came down from heaven.” He was asserting to those disciples that he was in fact God incarnate. That’s the first step to becoming a Christian, believing that Jesus is God, just as Jesus pointed out a few verses earlier,
“The work of God is this: to believe in the one he has sent.” So, when they start their grumbling that you pointed out, they are asserting their disbelief in His being God,
“Is this not Jesus, the son of Joseph, whose father and mother we know? How can he now say, ‘I came down from heaven’?”
So, you are correct, it was not ONLY the matter of eating his flesh and drinking his blood that the disciples were responding to when they said,
“this is a hard teaching”; it began with their utter disbelief that he was God.
He moves on from this pure figurative language when He asserts even more boldly than before,
“I am the living bread that came down from heaven.(vs 51)” To which they again assert their disbelief in His being God and, simultaneously, assert their disbelief that he can in fact, give us his flesh to eat,
“How can this man give us his flesh to eat? (vs 52)”
At this point, Jesus spells it out for them quite clearly,
“Very truly I tell you, unless you eat the flesh of the Son of Man and drink his blood, you have no life in you. 54 Whoever eats my flesh and drinks my blood has eternal life, and I will raise them up at the last day. 55 For my flesh is real food and my blood is real drink. 56 Whoever eats my flesh and drinks my blood remains in me, and I in them.”
So, at this point, Jesus has very clearly explained that He is God, and that his flesh is in fact food and he will give it to the world to eat. However ridiculous that sounds, it is what He said.
Now the grumblers, continuing with their disbelief of everything He said, respond, “This is a hard teaching. Who can accept it?”. Jesus, knowing them better than they know themselves, explains their disbelief to them,
“Does this offend you? 62 Then what if you see the Son of Man ascend to where he was before! 63 The Spirit gives life; the flesh counts for nothing. The words I have spoken to you—they are full of the Spirit and life. 64 Yet there are some of you who do not believe… 65 This is why I told you that no one can come to me unless the Father has enabled them.”
I take this last bit to mean that even if they were to see Him ascend to into Heaven that they would not believe because they are:
- selfish, “Very truly I tell you, you are looking for me, not because you saw the signs I performed but because you ate the loaves and had your fill. vs. 26”
- closed-minded, “How can he now say, ‘I came down from heaven’? vs. 42”
And
- closed to the Spirit, "The Spirit gives life; the flesh counts for nothing. The words I have spoken to you—they are full of the Spirit and life. vs. 63"
The things Jesus said were profound and required openness to many possibilities about God that these people had not considered and were not willing to consider. Most of all, as Jesus explained no less than 3 times in vs. 36, 44, 65, belief of these things required a work of God inside them - something they were not open to. They were too focused on themselves, their “flesh (vs. 63)”, tangible things, to allow the Spirit to give them “life”, understanding, and belief.
This most certainly was a salvation issue for those who were listening on that day. The culpability of those of us who were not there and may not have received the proper teaching is up to God, but you must realize, that for some, this IS, and for those it WAS, absolutely a salvation issue. Jesus said so. I’m not saying that anyone would go to hell over it, I’m in no position to make such a claim, just repeating what Jesus said, that “whoever eats my flesh and drinks my blood has eternal life”…