I have not read this whole thread so others may have commented similar already.
It seems to e that the whole basis for this data means that the only possible outcome is that people leave the "truth" in a shorter time more recently.
The reason is because your sample population is those that have left (even if mentally if not physically), i.e on this board. If someone came into the "truth" in the 1950's then they have a possibility of being a witness for 60 years or so, if they came in in 1990's then they could only do about 20 years but if they came in in 2010 they could only be a witness for a max of 5 years.
Granted some in the 60's would have lasted as a witness only a year or 2, but all of the 2010 would have only lasted a shorts time.
So obviously the later they came in the less time is was before they left.