God knew man would sin. Yes. But His perfect plan is not being abrogated. Man is being returned to the original position God placed Adam in before the fall. As Lafon said, it was a perfect situation without death, because God said it was VERY GOOD.
You noted that God is not reactionary. You actually contradicted your own hypothesis by saying that, though. You are claiming He would not have created man in the situation on earth that He did if He foreknew man would NOT sin. So He created by His foreknowledge of man's sin, when that cannot be the case.
God made man in the way and situation He did , and did not do so as the result of knowing man would sin. The foreknowledge of man's sin did not take God back, obviously. It did not make Him do things differently if reality was that sin would not occur and He knew it. He just had a means of restoring man perfectly back to the prefall status in which He made man.
If the absence of future sin was the reality, as we know it isn't obviously, God would not have created man in any different situation than He did. That's the only way we can define VERY GOOD and the only way to read the Word and follow God's will without adding to the word with speculation. It's Occam's Razor in full force.
We have to believe that God's situation for Adam was sub-par for us to think we are not being returned to that position. But God said it was very good.

Did God resort to PLAN B because PLAN A was not good enough? If man's destiny is different from man's situation in prefall times, then there are TWO PLANS of God which implies failure on His part the first time. Or, the only other conclusion is that God is reactionary which you admit cannot be the case.
With all due respect, brother, you are insisting on your hypothesis for the sake of upholding your belief that man is not intended to live forever on this earth free of death. You should be considering the points Lafon and I pointed out with the thought of why God put man in the situation He did, and what the Word described it as, rather than focusing on retaining your current belief of man's future. Your focus is on your belief instead of the actual Word presented to us. I know you claim you got your belief from the Word, but you are not focusing solely on it in the record of
Genesis 1-2 in this instance. You're remaining conscious of your need to not be wrong in your belief of man's destiny.
You still never told us where man will exist physically forever. Why is that?