Patrick:
Your analogy is apt in that they are both former sites of national open championships although Merion still packs a lot of punch as a test of golf whereas Prestwick, IMO, lacks the same degree of difficulty on its "hard" holes and has even more "short" holes than Merion - especially at the end.
I believe that Merion would hold up as a "test for the best" under championship conditions. One reason is that several of the longer holes can be extended (#'s 3,5,6,14,17 and 18) and even #12 has room for 10-15 more yards.
Unfortunately, and this is the rub, the tournament would have to be played with small galleries, modest TV and press capabilities and few corporate tents.
The Open Championship outgrew Prestwick's golf course 2 generations ago. The U.S. Open has outgrown Merion's facilities only since 1981.
The other similarity between the two is that they are both glorious places to enjoy all that is wonderful about golf.