Why doesn’t God totally remove Satan and all demons now, since he will someday without their approval (Revelation 20:10)?
Here is the answer I propose in the first paragraph of chapter nine of Seeing and Savoring Jesus Christ. The rest of the chapter gives the biblical basis and implications.
The glory of Christ is seen in his absolute right and power to annihilate or incapacitate Satan and all demons. But the reason he refrains from destroying and disabling them altogether is to manifest more clearly his superior beauty and worth. If Christ obliterated all devils and demons now (which he could do), his sheer power would be seen as glorious, but his superior beauty and worth would not shine as brightly as when humans renounce the promises of Satan and take pleasure in the greater glory of Christ.
By John Piper. © Desiring God. Website: desiringGod.org.