The Black Hole argument in regard to the NT accuracy

The "black hole" argument is basically since we have no extant original manuscripts from original authors, and only fragments in the 2nd century, and the earliest complete copy of the New Testament, the Codex Sinaiticus, which dates to the 4th century we cannot have any confidence as to what was originally written. The original author's words could have been changed prior to the 4th century, that "black hole" of manuscript data. source

If one looks at this graphic 

You'll see that there are 5 different families or lines of NT manuscripts; so if there were these changes in the texts during that "black hole" of manuscript data, why would all four families of texts, excepting for spelling differences and other minor changes, be basically be the same?

Note that these families are from geographically diverse areas; Egypt, Italy, Turkey, Palestine, Iraq. That's a lot of ground to cover travelling by foot. Highly unlikely that one person or group controlled or had influence in that wide of an area.

What theory best explains the essential agreement of all the NT extant manuscripts? 1) the copyists were very careful not to make changes [except for minor slips of the pen] from the start, or 2) copyists did make changes to the text including core doctrines up until the 2nd to 4th century and then stopped.

Also, for advocates of 2, I'd ask what was the catalyst for the stoppage of these alleged changes? Was it just coincidence that they stopped the changes then, did they know that is when we would have extant manuscripts?

That's very convenient for their theory. All these changes were being made, but we have no evidence [lack of manuscripts] but when we have evidence [a plethora of manuscripts] there are no changes.

Also, it seems that 2 is assuming, without any data, that major changes were made.

Conclusion/Thesis: the best explanation of the data is that those who took the time, effort, and expense to produce a copy of a NT manuscript were careful in the copying process, from the original authors down through the centuries.