I think this question reflects a common error in our thinking which I call temporal lock - that is, we have a great tendency to reason about the eternal things of God from a temporal and limited perspective.
God is the great "I Am" - the eternal present. God's eternity certainly is more complex than time continuing forever; it's timelessness. God is not bound by time; he does not exist within time as we know it - our time was created by him when he created the universe: Genesis 1:1 "In the beginning..." (emphasis mine).
It's clear from the laws of physics of this universe that it was never designed to exist eternally. The laws of thermodynamics ensure that the universe, much less this planet, cannot sustain life indefinitely. Equally, consider what was to happen once mankind had fulfilled the command to multiply and fill the earth - then what? It seems apparent from the creation itself that this entire universe is an interim endeavor. While it is feasible that God could have stepped in and done something supernatural, it's more reasonable to postulate that this creation is temporary and is designed for a purpose, which once achieved, will negate the reason for its existence.
It's not so much that mankind would inevitably sin, but that in God's experience, there is no other reality. God knew from all eternity that man would sin and what he would do about it - the plan of salvation was in the heart of God before (whatever "before" means in God's dimensional existence) this creation ever was.
If one reasons this through to its logical conclusion, in the face of a perfect eternal being, one must conclude that this creation, the fall and Christ's redemptive action is a necessary interim step in God's ultimate plan for our eternity. An eternity in which mankind has perfect communion with God, has free-will and the capability to love, and yet is incapable of sinning.