Mankind was originally created sinless as well, however, the temptation was for a state preferable to remaining in sinless communion with God, being your own God, this why humility and selflessness is a big theme in the teachings of Jesus and Paul.
I guess we're also assuming in this thread that the post-exile understand of Satan as a fallen angel at opposition with God is the correct one, and not the pre-exile understand of Satan as God's, for lack of a better term, District Attorney? Since we are, Satan's rebellion of God was completely different than the sin of Adam because angels are immortal, spiritual beings of a completely different purpose, role, and etc. When Satan, along with 1/3 of the angels, those chose were the only ones effected, just like damned men choose their sin over God. Another important factor, how do you know with 100% certainty there is no plan for fallen angels to be redeemed? Why would scripture even mention that as it as no relevancy to the relationship between God and man because that's an issue between God and his angels.