Yes, it would have been great to see the first grade final played before the Bledisloe. One game would surely have little effect on the surface, especially after a prolonged period of relatively dry weather.
However, the lower grades would still obviously need to be played elsewhere.
Arrangements would have had to be made to allow reserves to be available for the main game, if one of the first grade grand finalists also had a team in the reserve grade grand final (could have happened for the Woodies this season).
Finally, only people with tickets to the Bled would have been admitted to the ground, ruling out a number of loyal club supporters who either did not want to go to the Bled, or could not afford it.
So it would have been a nice idea, but there are some serious obstacles.