First of all, Mangum can scramble. Quite good at it actually. Very mobile. Hill didn't scramble to pass. He did it to run. So it's overrated if part of the reason you're running is because of what you lack as a passer.
When things break down, it should be easier for Mangum to find RBs and TEs for a quick pass. Hill rarely saw either of those two, and its going to be a planned part of the offense anyway.
Of course they'll miss Jamaal. I can't find any reason why they'll miss Hill. Just is what it is unfortunately (and fortunately for the future).