Mueller's gig is to 'investigate' everything and anything in order to block any incriminating evidence on the bad guys being used in court via the 'ongoing investigation' canard.
IOW, even if POTUS has evidence on the bad guys he can't publicize it under the threat of 'obstructing justice'.
Reducing the scope of Mueller's power could result in the incriminating evidence he currently under his lock and key being freed up.