I think that Flutie had some good years but looking at it 1999 probably wasn't his best:
75.1 passer rating
19 TD's - 16 INT's
6.6 YPA
Bills averaged just over 19 points per game
Those are not good numbers.
The one number everyone points to:
10-5 win-loss record.
However, the defense gave up a total of 110 points in those 10 wins. 11 points allowed per game is nice support.
In contrast which QB in Bills history had this stat line?:
84.9 passer rating
19 TD's - 14 INT's
7.1 YPA
Team averaged 18.75 points per game
Hint: It's 2006 JP Losman
I think that Flutie wasn't great in '99 and it made this decision much easier. As for winning the game going away I don't think Kelly in his prime wins that game going away. He might win it but let's remember the Titans were a 13 win team that year not a bunch of stiffs.