Rielly is on the top PP unit because he is the better PP option. It has nothing to do with the amount of money he makes.
People really overrate the factor of a shot on the PP, short of it being someone like Makar or Josi, and the Leafs would need to change their system to actually use their defenseman more actively as a shooter on the PP if we want anyone to be a factor.
Rielly has as many PP goals over the past 3 years as Hughes and Fox. And he played 20+ fewer games than them too. Giordano has had 6. Those 3 extra goals over 3 years are not putting the PP over the top.
If we really want our defense to get more involved, maybe allowing to shoot at a time beyond near the end of their shift would be helpful. Even last game against the Habs, there were more than a couple of times when both Rielly and Gio could have let a solid wrister off for at least a tip by one of the two guys in front of the net but instead passed it off to a winger and the PP was kept to the perimeter for the most part the entire time.
Beyond that, he is 39 years old. If we get to where we want to go, and he plays a full schedule, he would have to play triple digit games this year. I would rather he sit some games and be able to sustain his high level of play in 20 minutes a game than to run out of steam at the end of the year when it really matters... And that will almost certainly happen as it does with everyone else. Maybe we won't have that luxury, but if we do, we certainly should be giving him as much rest as we possibly can.