What purpose does that serve them? They end up with a player sitting at home wasting away and still get even less crap for him.
Let's not forget McGroarty wasn't even the Pegs #1 prospect, but Yager was clearly the Pens. The Peg could afford to wait if they wanted to, but Yager brings a better much more desirable center attribute timeline for them. (the Pens as well) McGroarty is obviously seen as a winger both there and here. So that ups the value a good bit in Yager's favor. So being close in NHL production isn't the same. Center>>>wing every single day. Center's are hard to come by and they just gave up a good potential one.
McGroarty would have to be at Jake levels to wash away Yager's center attribute. A mere 20G/20A - 40/50 point winger isn't going to cut it if Yager is remotely the same to better at center. For one, you can move forward with Yager as one of the top 2 center positions, McGroarty would need such a center in a few years they likely won't have with all of them aging out. Which brings that position all the more important moving forward. Openings both at 2C/3C. 1C a mere year after that.
McGroarty would seriously have to fast track to be anything remotely useful in Sid/Geno's final years together. Right now, that's 2 seasons.
Right now people are already slotting him as Sid's LW this year. That's just not how it works normally.