2 cents from someone who took CS61A in scheme and had a hand in creating the new python version of the course and subsequently was a TA for the class for a few semesters on this debate:

In any class of reasonable difficulty/usefulness, there will be students who do great, ones that do okay, and ones that struggle. What we should aim to do as the very first intro course is to help as many of those people from all of those groups to do as well as possible in their future classes/careers.

For the great students, I actually believe we did them a disservice. CS61A in scheme was truly beautiful. It was amazing, given how simple the language was, how much you could achieve. All pieces of the puzzle fit together perfectly. Trying to do the course in python, on the other hand, we had to compromise a little bit. We didn't want to just teach the exact same content in a different language. We wanted to teach the "Pythononic" way of doing things. To give a few examples:
- The first 1/3 of CS61A in scheme had no mutation. That is almost impossible in Python to do, and is not the way the language is used in industry. I personally believe functional programming and immutability is The-Way to program, so I see this as a huge loss.
- We teach recursive lists (linked lists) in order to teach recursion, but I feel like having both the default Python mutable lists (which are array lists) and linked lists is confusing to students, since we do not teach their performance tradeoffs (that is in 61B).
- Mutual recursion was almost useless to teach. Where it was most commonly used in Scheme, you achieve the similar thing using list comprehensions in Python (the language feature essentially took away your need to construct the list in a recursive fashion).
- The very interesting bits of the old class about metacircular evaluators, we simply could not do in Python.
- Python also has tons of quirks (magic methods, for instance) and lost some of the elegance we saw in scheme.

But for the okay and struggling students - 70-80 percentile and below, the students who understand most of the material, but perhaps fail to grasp some of the more complex concepts - I think Python was the correct way to go for a few reasons:
- It is a much more commonly used language. It sets you up to go for internships, research, hackathons, whatever you please, straight from what you learned in class.
- The language is way more similar to Java, which allows students to translate what they learned easier to the next course in the series, CS61B.
- 80% of the material hasn't changed but you've gained the above benefits. Remember, we are talking about students who probably would have struggled to grasp those last 20% anyway (maybe they are completely new to programming and they already have their hands full with the 80% to begin with).

I think therein lies the difference in opinion. If you took CS61A in scheme and understood it very well, then you don't understand the change. If you understood the intricacies of what was taught in CS61A, you will find it very easy to generalize those concepts to new languages - and new concepts. In fact, you will find that much of what you learn in the rest of your undergrad career will be "CS61A review". However, for students who may not have had the CS maturity or time to grasp all the concepts, they find that the Python course is much more useful.