Well, you're correct that many of the originals were tampered with, and you're correct about the early Christians having an open canon; however, after Jesus' resurrection, he appeared to the apostles and many disciples. The apostles also organized congregations, ordained bishops and attempted to standardize doctrine (as the epistles indicate). But in those days the news was bleak:
In a standard 40-day situation the apostles, deeply worried, ask the Lord what lies ahead for them and their work, and receive an appalling reply: They are to be rejected by all men and take their violent exit from the world, what time corrupters and false shepherds will appear within the church, where a growing faction of the worldly-minded will soon overcome and annihilate what remains of the faithful saints. The sheep turn into wolves as the Wintertime of the just settles down the lights go out and the long age of darkness begins under the rule of the Cosmoplanes, disastrously usurping the authority of Christ. There is indeed a promise of comfort and joy, but it is all on the other side and in the distant return of the Lord. The apostles protest, as we do today: Is this a time for speaking of death and disaster? Can all that has transpired be but for the salvation of a few and the condemnation of many? But Jesus remains unyielding: that is not for us to decide or to question. The grim picture is confirmed by the Apostolic Fathers, who are convinced that they are beholding the fulfillment of these very prophecies, and are driven by a tragic sense of urgency and finality. After them the early patristic writers accept the pattern with heavy reluctance, and only the surprising and unexpected victory of the church in the fourth century enable Eusebius's generation to turn the tables and discredit the whole pessimistic tradition. (Dr. Hugh Nibley, Evangelium Quadraginta Dierum: The Forty-day Mission of Christ—The Forgotten Heritage)
Later churchmen placed the apostasia ahead of them and looked with dread for the fulfillment. As for the apostasy itself, it did not come from the outside in, but from the inside out. Congregations began vying with the apostles to elect their own bishops rather than have them appointed from the top down. There also were disputations about basic doctrines like the resurrection and the atonement. One scholar writes: "By the early second century, Christianity had fragmented into dozens of splinter groups with each group charging that the other possessed both forged and corrupted texts." In other words, when the apostles were dead and there was no more inspired leadership, wolves entered the fold, not sparing the flock.
But does this undermine original Christianity? We know that whatever state the Christians were in in the first century, the apostolic leaders were able to warn them to get out of Jerusalem by revelation and "stand in holy places" like Pella, to the north. When the Romans caught the Jews wholly off guard and quickly surrounded them, the Christians were long gone.