"Why is Satan loosed after the 1000-years... what's the purpose?"
As for those living in the seventh millennium, they will have nothing to complain about as far as salvation is concerned or otherwise. This is because they will have had one thousand protected years to move into perfection and be "changed in the twinkling of an eye." Of course for the saints that finish the work of salvation by redeeming the time (those who will be already redeemed), Satan's last ditch attempt to deceive them at the end of the thousand years will be to no avail. Nevertheless, it says Satan will be able to gather his ranks from outside both "the beloved city" and "the camp of the saints about": Revelation 20:9. Revelation 20 puts it this way.
"And when the thousand years are expired, Satan shall be loosed out of his prison, And shall GO OUT [of his prison] to DECEIVE the nations which are in the four quarters [meaning the whole] of the earth, Gog and Magog [meaning especially those of that linage], to gather them together to battle: the number of whom is as the sand of the sea." Revelation 20:7-8
At this point God's justice pertaining to mans' salvation will have been satisfied for all. Thus, even though there were many different dispensations, all were afforded a pathway to salvation and deliverance. God will have made "a Way" for all to be equally judged according to their situation. Additionally, one must consider all the souls who died as babies, the very young, and the mentally impaired; God knows how to factor in righteous judgment. Therefore, by taking a brief look at the big picture, we are able to see how justice was served to all these generations. The key factor for determining their judgment was and always will be accountability.
"Therefore to him that knoweth to do good, and doeth it not, to him it is sin." James 4:17
Still, some may cry "No fair!" when the seventh millennium Christian makes it into God's Kingdom without ever having to resist Satan because he was bound in their life times. So to be just, God will let Satan loose for a little season to try those who walk the Earth. As for those that die during the millennium, they will have no excuse either; they will have rejected The Blood without Satan's enticements! Thus, it is those that are still alive/mortal at the end of the thousand years that Satan will be able to recruit into his "Sand of the sea sinner army." Brethren this is why we must deal with all sin in our lives, or we too could yet end up in this very last day army of malcontents and complainers.
Another point that needs to be addressed at this time is the fact that some Christians have been taught that these sinners that attempt to assail God's saints at the end of the next thousand years, will have actually "come up out of Hell" with Satan. I believe this is taught because some believe there just cannot be any rebellious people living in the seventh millennium, and especially at the end thereof, but in fact just the opposite is true. This premise is well established in the book: "The Seventh Millennium...." However, for those who have not read it yet, let me just mention one addistional point. If someone has died and is already in a hellish realm because of rejecting the love of God (the sacrifice of Jesus' death on the cross for their sins), then they are and were already DECEIVED, that is what got them there! It is very important that you understand this basic truth. Even if someone could come up out of Hell in bodily form to live again (which they cannot: Heb 9:27), the fact would still remain that they were already deceived, having died without the blood. How is it then that the Bible says these "people of the nations," these "sand of the sea sinners," are going to BECOME deceived once Satan is loosed? In essence, Satan is just repeating once more that which he has done before (Jude 1:6): "How art thou fallen from heaven, O Lucifer, son of the morning! how art thou cut down to the ground, which didst WEAKEN the NATIONS!" Isaiah 14:12