It's not bad to give all kids the "gifted" classroom. It's just impossible. There's not enough "gifted" resources to go around. It's a zero sum game at some level - to give "gifted" education to someone you have to take it away from someone else.
The whole point of this is that the teachers were not special teachers. The students were not special students. But the joint exercise they did was new and interesting. It is more about changing teachers' attitudes and skills than about stratifying resources and kids into gifted and not-gifted.
You might be saying that sea changes are impossible due to entrenched interests in education. I do think some reform there is very important. But I also think that as administrations and states see the benefits for teachers and students, they will start gravitating in this direction.