Excellent question. I have studied this very issue for a very long time. The way I understand it and believe is that, indeed the KJV is the more sure Word of God. It is the God promised 7th level of purification, tried in the furnace of earth.
II Kings 12:6
The words of the LORD [are] pure words: [as] silver tried in a furnace of earth, purified seven times.
Here is a list of versions leading up to the A.V. 1611 text
German Luther Bible
No, the problem, from what I see is that not that the Christians of the past getting it wrong. More to the fact of why anyone would mess with the Word after that point, is the question, really. Do you know of all of the deletions that have occured since that point? Why do you think anyone would take the time to rewrite the bible and make it more complicated? Is God the author of confusion?
Here is a list of just the New Testament deletions, I do not know about you, but when people go about deleting entire verses, it really makes me think why.
Matthew 17:21, Matthew 18:11, Matthew 23:14, Mark 7:16, Mark 9:44, Mark 9:46, Mark 11:26, Mark 15:28, Mark 16:9-20, Luke 17:36, Luke 23:17, John 5:4, Acts 8:37, Acts 15:34, Acts 24:7, Acts 28:29, Romans 16:24, I John 5:7.