You needn't be a math expert, but you do need to read the entire exchange in context.
When I assert that Smith "provides the best possible fix for the next two seasons," I'm accounting for the ages of both Smith and Owens, who will be in his late 30s after two years--the age that most receivers show a noticeable decline.
In my very next sentence, I make a similar statement regarding the pairing of Williams and Owens: "Acquiring Williams provides potentially the second best possible fix for the next two years and the best possible fix for the next seven or eight years." Once again, my statement accounts for the advancing age of Owens, and then considers the prospect of life post-Owens.
I never stated that Williams will play 5 to 6 years longer than Smith; I simply think Williams is more likely to play 7 or 8 more years than Smith for two reasons:
1) Williams will be 33/34 in 7 years while Smith will be 36/37.
2) Receivers such as Roy Williams, whose game is predicated largely on size and strength, seem to play at a higher level for a longer time period than receivers such as Steve Smith, whose game is predicated on speed and quickness. Case in point: Terry Glenn's career is probably finished while Owens is still playing very well.