He MUST increase. Isaiah tells us that there will be no end of the increase of His government and peace. In the beginning was the Word, and we can see how God has worked steadily from the beginning to increase Christ. From types and shadows in the Old Testament we see Christ coming into view. Then the Word is made flesh and dwells among us, and Christ is increased yet again. Next He comes to dwell within us, and this is a major increase. Finally, He begins to conform us to His own image through His indwelling Life.
If we are growing up into Him then He is increasing daily. Eventually every knee will bow and every tongue will confess that Jesus Christ is Lord. Beyond this, we are told that God will continue to reveal His Son in the ages to come, bringing us into depths and dimensions of Christ that we cannot fathom.