You assume Jesus was talking about the final coming of the Kingdom but since that didn't happen we know that is not what he was talking about.
Well, that made me laugh out loud.
I am infallible. I predict that today an elephant will fall on you. If that doesn't happen --- well, I'm infallible, so I must not have meant "an elephant will fall on you"
in the sense of "you will be the subject of the unchecked descent of a pachyderm". In that case we know that I must have meant something else, and you may speculate on what.
Of course, if all my statements, no matter how apparently plain and simple, may in fact be obscure cryptic puzzles for the delight of hermeneuticians, then there's hardly any point in my making them. And the same applies to Jesus. If, by "the Son of Man coming in his kingdom" he didn't mean the Son of Man coming in his kingdom, then what reason do we have to think that when he said "love thy neighbor as thyself" he meant that you should love your neighbor as yourself? Maybe he meant that you should buy a bassoon and play it aggressively at squirrels.