Do you tell ALL the drivers to slow down? Even the ones not on Michelin tires? What happens when a Bridgestone-shod car comes through that corner 50-100mph faster than a Michelin car? Who moves? That'd create a very dangerous situation.
If memory serves, drivers in one the Daytona 500's from the late 80's were faced with this exact situation. Hoosier had supplied a tire that enabled most of their teams to qualify up front and even put one on pole. It was subsequently found to be too dangerous for the race and NASCAR allowed those Hoosier teams to use Goodyear's IIRC.
Michelin is completely to blame for not supplying a tire suitable for competition. However, the FIA is to blame for not reaching a compromise so that there still might have been a race.
