I believe Muhammad was a false prophet for many reasons. Primarily, the fatal mistake of his narrative is that the Islam is contingent upon the Christian mission being a failure. According to orthodox Islam, the nondescript Messiah of Islam (merely a prophetic man with a special end-times role) had his mission immediately thwarted by the incompetent disciples, who twisted Isa's purely Islamic message into human-worshipping idolatry immediately. Even if recourse is had to some fringe group of 'true Christians' surviving, like the Ebionites, it is impossible to deny that these 'true Christians' were utterly destroyed by the tidal force of the Apostolic Church, which rejected them immediately as apostates of the Christian faith. In the Islamic view of history, the long-awaited Messiah came, 'appeared to be crucified,' and ascended into heaven after putting into motion a plan which failed less than a single generation after the Christ's birth.
Why would Allah enact a plan like this, where the disciples of Jesus immediately fail to uphold "true Christian faith," when Allah himself allegedly gave this promise to the disciples:
"O ye who believe! Be ye helpers of Allah: as said Jesus the son of Mary to the Disciples, "Who will be my helpers to (the work of) Allah?" Said the Disciples, "We are Allah's helpers!" then a portion of the Children of Israel believed, and a portion disbelieved: But We gave power to those who believed against their enemies, AND THEY BECAME THE ONES THAT PREVAILED." - Sura 61:14