The issue becomes add 1 slow lap and a bit time on so everyone can get in one more hot lap, everyone will be on used older slower tires. With people in various stages of their laps when they know they have to back off going again will have very unfair tire life going on. You could provide everyone with a spare set though that is a lot of extra tires to have around just in case. Could say it is only done for Q3 because it's more of a disappointment to finish qualifying with everyone pulling out of their laps. Who do you cut off, what if three people finish their lap, one goes pole but the track condition is improving due to increasing/decreasing temp, rain drying out. 15 mins to clear a track, they all go back out and suddenly those who got on pole aren't after another run?
There isn't really a fair way to go about it.