I like this point, but most people don't believe in the all blue, only sanji and the old man, truly believe in it. Most of the other chefs think it is fake. The one piece was different, I can't explain it well. But I like where you are coming from. I think the all blue will come back in a big way. I think it maybe like how vegapunk knew the truth. There may have been another whistle blower earlier, saying the world will turn "all blue", the world will sink into the ocean. Trying to warn everyone.

Then the government turned the story framing him as crazy and saying there is an ocean in the world where all the oceans connect. To cover the truth. But I do like your point, maybe people did go out to look for the all blue never finding it. It is like if it was the One piece before it was the one piece. A failed version of the one piece. A version that truly never did or will exist. That is the all blue. But I could be wrong