But, you could still implement car and cdr in terms on conc, so this is not really an issue.

It might still be an issue. The more distance there is between how code is represented and how data and lower levels of code are represented, the less fluid programming in Lisp becomes. Yes, you can translate one to the other. But the sweet spot of Lisp is the space where no such mapping is needed or it's trivial; the magic drops off pretty sharply as you leave that space. It's not obvious (to me anyway) what the effect of what we're talking about would be. Small changes can have surprisingly big impacts, and one has to include the psychology of this.