Shoot, if he played 5 or 6, that's probably enough to gain a great deal from.
But in the end, to heck with Hasslebeck. The goal OUGHT to be to win games; not to potentially give up wins in order to prep a new QB.
So if Keenum is playing very well, then Goff needs to sit.
And since Fisher is a player's coach, you already know that he's not about to create team dissension (which it would), if they're playing Keenum and winning, by deciding now's the time for Goff despite Keenum's effort. It would send a bad message to the team. Get drafted high and that makes you the starter regardless of what happens on the field. Like I said: that's not Fisher, anyway.