Good point, Jason. I was expecting someone to bring that up.
Merion is a brilliant design very much dictated by the land constraints, somewhat like Philmont with its pre-existing course and clubhouse. Merion's routing is unique in golf. To me the routing progression is best looked at in groups of six rather than nine. If you look at the first six holes there is one long par 3, two long par 4s (5 and 6) 2 long par 5s (2 and 4) and a lone short par 4 (1). That's a long and difficult start.The middle six holes consist of a mid-length downhill par 3 (9) and four short par 4s (7,8,10 and 11) and a longish uphill dogleg par 4 (12). The last six holes are a mixed bag starting with the short par 3 13th balanced by the long par 3 17th. The long par 4s (14 and 18) counterbalanced by the shortish par 4s (15 and 16). For some reason that is difficult to put into words, the Merion routing progression is perfect. It played differently for a time prior to the 1916 Amateur with the current 3-7 playing as 7,5,6,3,5. It doesn't work as well. I've tried it.
Now if you think of the collection of holes at Philmont, I don't think the flow of the holes is very good today but I fail to see how it is improved with a new routing as I suggested it might be. I need to think about this more and maybe I'll come up with something compelling. For now it escapes me.