As it happens, I agree on this point.
The problem with believing that Jesus existed but was not the Messiah is that, if he was real, he obviously had followers who believed he was the Messiah. The question then is why did they believe that?
Josephus mentions several men from that era who were thought by some to have been the Messiah. All of them led rebellions of some sort against the Romans. When their rebellions failed and they were killed, they were judged in retrospect not to have been the Messiah.
As far as I know, there is no historical record of a rebellion led by a Jesus of Nazareth. What could a 30-something man like Jesus have done to make people believe he was the Messiah? His sermons as recorded in the Gospels are eloquent but not earthshakingly profound. Do you think he really performed the miracles (mostly exorcisms) he is credited with in the Gospels? Do you think he walked on water?
He may have performed healings, as did other itinerant charismatic preachers of the time. The other miracle stories were late attributions.
What could a 30-something man like Jesus have done to make people believe he was the Messiah?
In addition to his more customary public preaching, he taught an apocalyptic message which was reserved for his small inner coterie of followers. This apocalyptic appears to be related to Essene teachings. In Jesus's messianic vision, an army of men would not be needed ("my kingdom is not of this world"), because the battle would be fought by an army of angels.
So it will be at the close of the age. The angels will come out and separate the evil from the righteous,
and throw them into the furnace of fire; there men will weep and gnash their teeth. (Matthew 13:49-50)
For the Son of man is to come with his angels in the glory of his Father, and then he will repay every man for what he has done.
Truly, I say to you, there are some standing here who will not taste death before they see the Son of man coming in his kingdom. (Matthew 16:27-28)
For as the lightning comes from the east and shines as far as the west, so will be the coming of the Son of man. Wherever the body is, there the eagles will be gathered together.
"Immediately after the tribulation of those days the sun will be darkened, and the moon will not give its light, and the stars will fall from heaven, and the powers of the heavens will be shaken;
then will appear the sign of the Son of man in heaven, and then all the tribes of the earth will mourn, and they will see the Son of man coming on the clouds of heaven with power and great glory;
and he will send out his angels with a loud trumpet call, and they will gather his elect from the four winds, from one end of heaven to the other.(Matthew 24:27-31)
This was a mystical vision of messianic triumph, with the Messianic Age coinciding with the End of Days. Jesus seemed to have arranged his last days to fulfil his interpretation of prophecy (his arrival in Jerusalem, for example). It is possible that he even arranged his own betrayal, entrusting Judas with the duty to report his whereabouts to the Romans.
Jesus answered, "It is he to whom I shall give this morsel when I have dipped it." So when he had dipped the morsel, he gave it to Judas, the son of Simon Iscariot. Then after the morsel, Satan entered into him. Jesus said to him, "What you are going to do, do quickly." (John 13:26-27)
I think that their expectation was that, at the moment of tribulation, when he was crucified, an army of angels would appear, and bring about the end of the age. Given the actions of the apostles, it appears their expectation was that this army of angels would rescue Jesus before or immediately upon his death. When he actually died,
My God, my God, why hast thou forsaken me?
his apostles scattered.
It is equally possible to make a case that his death was anticipated, and that he would be resurrected "very soon" to lead the angelic army.
I tell you I shall not drink again of this fruit of the vine until that day when I drink it new with you in my Father's kingdom...
Then Jesus said to them, "You will all fall away because of me this night; for it is written, `I will strike the shepherd, and the sheep of the flock will be scattered.'
But after I am raised up, I will go before you to Galilee." (Matthew 26:29,31-32)
I think the prior is more likely, and that his actual death led to a reappraisal of their expectation. In any event, given the mystical nature of their expectations, it is not surprising that his core of followers had visions of him alive ('resurrected')
But he, full of the Holy Spirit, gazed into heaven and saw the glory of God, and Jesus standing at the right hand of God;
and he said, "Behold, I see the heavens opened, and the Son of man standing at the right hand of God." (Acts 7:55-56)
Whether they thought it would happen at the moment of his death, or three days later, when the expected messianic moment didn't come, they undoubtedly were discouraged and questioned what went wrong. My guess is that at some point, one of the apostles had either a vision of Jesus resurrected, or came in some other way to the belief that Jesus was in fact alive and would return soon to finish what he had begun. And the rest is history.