The problem is pro-punishment politicians. Singling out any particular office/position misses the point.
Even when judges practice discretion, pro-punishment legislators step in and introduce mandatory minimums.
And even when prosecutors practice discretion, pro-punishment judges will step in and force them to prosecute cases that the prosecutors don't want to prosecute [1].
[1] See Suffolk County for a recent example
And in many places prosecutors are politicians in a sense, because they are elected. One recommendation is to go back to appointing prosecutors rather than electing them.
So no, singling out prosecutors doesn't miss the point. It is an important point.
The problem with focusing on any particular role is that the other parts of the system will step in to force punishment.