That's an interesting interpretation. One might then suggest that McDavid and Drai are benefitting in the same way, and that Barrie's numbers, (one year here, Colorado for one and his 3rd in Edmonton - lucky fellow that), has had his numbers substantially inflated due to playing with all the offensive talent from 3 teams. Tyson Barrie has feasted on the PP: 6th in overall points with .3 points per game, at 3:17/g he's 32 seconds more per game. Or more methodically, .026 more points per game than Morgan. That, over 82 games, measures out to 2.1 points more for the player per year. Draw your own conclusions. But back to McD and Drai for a second. They are 1 and 2 for scoring on the PP over the last 5 years. Auston and Mitch: 18 and 24, with JT sliding in at 28. However, combined we're still lagging the point totals from our fellas that work the "magic" when compared to the duo from Edmonton. McD and Drai score more than our top 3. Where's the magic? One could argue we had one of the most dominant PPs of all time, last year. And they'd be right! We did. So why don't we score more? I'd argue, and please forgive me as I don't have the reference to back up this truth, the Leafs receive fewer PP opportunities than just about anybody in the league. I'm sure I remember that Colorado is number 1, but almost 100% we receive fewer PP than any other team. The reasons, I'll leave to others, but Morgan Rielly's perceived effectiveness - however you wish to measure from what I can find - is largely based on the fact that he has fewer chances to shine, and in fewer minutes to do so, than just about any top tier PP defenseman in the league. Tangible results, points, time on ice, opportunities, MATH...he ain't the best, but we could do a lot worse.
Edit: There is no evidence that Sandin and Lily, and I like both, would be an improvement over Rielly on the PP. There is hope they'd succeed, but no numbers they have produced to date in the NHL suggest that they'd by anything but a downgrade. Nothing.