Leafs PP fell to last place in the league at 14%
Of the 314 seasons including this one by teams in the league since 2016-17 (when Matthews/Marner arrived), this season so far by the Leafs would rank 12th worst (303rd of 314 team seasons in the league since 2016-17)
Matthews, Nylander and Tavares are in the top 27 in PP scoring out of ~1350 forwards who played during that time.
Rielly & OEL are around top 27 in PP scoring out of 600+ dmen who played since Rielly came into the league.
With that kind of proven talent, the result is that much more incredible. They appear to have the talent - though some of it may have regressed with age/injuries ...
So I looked at one other thing. The Leafs scored 474 PP goals during the seasons Marner played for them. Marner scored or assisted on 228 of those goals, leading his team in PP scoring - in on 48% of those PP goals. Marner was their leading PP scorer in the playoffs too - in on a whopping 64% of those PP goals. I think part of the issue has been demonstrated as they're missing Marner's DNA on the PP and haven't figured out or solved how to replace it. Didn't see much of that DNA in the acquisitions Treliving brought in. And they have failed to adjust or compensate without it.
So it seems arguably unfair to blame this all on the coaching.