Did the Chelios hit really impact Propp that much? His stats did go down a bit in 1989-90, but he was 30 by then, a not untypical age for forwards of his time to start slowing down a bit. The Flyers were also in a decline by then, so I don't think he was the only one whose stats suffered.
I don't blame Propp, of course, for his team's losing those series, but the question is more one of how did he perform in those series? I remember him being pretty good in the '87 Finals, but I wasn't aware of him in '85 (I was too young), and he didn't make any impression on me later on. (The Pens weren't a dynasty, btw.)
That's not correct. Gretzky had 22 points in two rounds of playoffs in '89.
I like Propp, but I think you might be over-stating the significance of the Chelios hit. Of course a concussion is serious, but Propp had 4 or 5 months, at least, to rest and recover after it. (This is assuming it was a fairly serious concussion -- I don't know how bad it actually was.) That hit may have had some lingering effect, but more likely Propp was just passing his prime years at the same time the Flyers were entering a decline.
As Propp is the classic 'borderline' kind of Hall-of-Fame Candidate, I think the question comes down to: Was he the kind of player who, besides his borderline Hall of Fame stats, could elevate a team towards victory? His teams' records in the Finals suggests not. (Note: Don't take this as a criticism of Propp -- the same applies to several of my own favorite players. Example: Bernie Nicholls.)