For God to eliminate Satan (such as to snuff out his existence) would be contrary to God’s own will, as God created the angels as immortal beings.
For God to eliminate Satan (such as to remove his ability to act in creation) would be contrary to God’s own will, as God created the angels with free will.
As Victor mentioned, there would still be evil in this world even if the devil and all his demons were sequestered away. Man is capable of any evil apart from outside influence.
God’s plan was to give us a choice between good and evil. We know this because He gave us free will. However, do not make the mistake of identifying Satan as the sole or supreme source of evil. Satan, though an angel, is still nothing more than a mere creation. We are monotheists, not dualists (a system wherein a supreme power of good is opposed by a supreme power of evil – a number of people mistake Satan for such a power).