Is it true that if there were no literal devil, there would have been no need for Christ to appear?

Answer:

"Little children, let no one deceive you. He who practices righteousness is righteous, just as He is righteous. He who sins is of the devil, for the devil has sinned from the beginning. For this purpose the Son of God was manifested, that He might destroy the works of the devil" (I John 3:7-8).

The work of the Devil is sin and Jesus came to destroy sin. That is what John also said a few verses earlier, "And you know that He was manifested to take away our sins, and in Him there is no sin" (I John 3:5). Jesus came to take away Satan's full power over the world. "Having disarmed principalities and powers, He made a public spectacle of them, triumphing over them in it" (Colossians 2:15). Through sin comes death, so we also learn: "Inasmuch then as the children have partaken of flesh and blood, He Himself likewise shared in the same, that through death He might destroy him who had the power of death, that is, the devil, and release those who through fear of death were all their lifetime subject to bondage" (Hebrews 2:14-15).

It is hard to discuss what might have been when there is no doubt as to what is. I don't know if sin would have entered the world through some other means if Satan wasn't there. The real point is that Satan has been around since the beginning. I assume you are referring to this verse as evidence the Satan must really exist, yet the Bible never refers to Satan as an illusion. I would think Jesus' temptations and his resulting conversations with Satan would prove that point (Matthew 4).