When Jesus said, "Behold, I come quickly," he didn't mean he would return at that time 2000 years ago. He just meant that, when he comes (returns), it will be quick, not taking very long to unfold.
And when He said "This generation shall not pass away," I've always felt that he was talking about when Israel became a nation again back in 1948. And there are lots of people from that time still alive.
Edit to add:
Did you know that if you count the years from Adam up to Abraham, whom God told Abraham that he would be the founder of a great nation, those years add up to be 1948 years? Coincidentally the same year that the nation of Israel started again.
And when He said "This generation shall not pass away," I've always felt that he was talking about when Israel became a nation again back in 1948. And there are lots of people from that time still alive.
Edit to add:
Did you know that if you count the years from Adam up to Abraham, whom God told Abraham that he would be the founder of a great nation, those years add up to be 1948 years? Coincidentally the same year that the nation of Israel started again.