But you can also argue that they had competing ministries.
At one point in one of the Gospels, the disciples are angry that Jesus’ disciples are baptizing folks, which they thought was exclusively John’s territory.
The disciples did not know of John’s role and how he fit into God’s plan. Jesus had to explain things to the disciples.
John the Baptist is a bridge between the Old Testament and the New Testament, between the Holiness Code of Moses and the Law of Love of Jesus, between the Old Covenant and the New Covenant, and between the old kingdom of Israel and the new Kingdom of Jesus.
John is the last of the Old Testament Prophets. Scripture stated that Elijiah would come again before the Messiah came. Jesus explains that John is Elijiah and that he is the greatest of all the prophets. John is in fact the last of the Old Testament prophets. Jesus goes further in telling the disciples that although noone is greater than John the Baptists, those under the New Covenant will be infinitely greater than he.
As an Old Testament prophet, being a disciple of Jesus was not part of God’s plan. In saying that he must decrease, John is anouncing the end of the Old Testament prophets. Many of the Old Testament prophets were killed for what they said, and as the greatest of the Old Testeamnt prophets, John will be no different. But in saying that Jesus must increase while he himself decreases, John is opeing up the way for the New Covenant, “Making straight the way of the Lord” as it were.
What John says is profound. I must decrease - the Old Covenant and old nation of Israel is going away. He must increase - the New Covenant is about to be introduced and a new Kingdom established. And the new kingdom will be small, hidden, like the mustard seed in Matthew 13, but it will grow into a large srub and a tree (mighty nation) and the birds (Gentiles) will make nests in it.
It was not in God’s plan that John “increase” or be an apostle. John is the very last of the OT prophets, the end of the age of the Old Covenant, and so he must decrease at the advent of Christ.
-Tim-