I read interview of Hal Abelson, in that interview he explains the reason behind moving away from scheme to python to teach concepts of SICP.
1. The purpose of course it self has changed, the current course 6.01 in MIT attempts to introduce grads to breadth of the software engineering rather than depths of software engineering. 6.001 course was for the later purpose.
2. He says entire debate is superficial as both the course have different purpose.