Do you always quote yourself when you ask questions?
I think the argument could be made that at the time of discovery, MPFW was THE most expensive comic (the incomplete copy sold for $2000 in 1974, far more than Action #1 or Marvel #1 guided for in good).
I believe it immediately ranked so high because 1) Bob bought the very first copy sold 2) It was the first Sub-Mariner (and some suspected, by extension, kind of the first Timely), 3) There were no comparables to shop, if you wanted a MPFW you had the beg the guy who had all the known copies (and he apparently misled people as to how many copies there were), you couldn't find one from another dealer and try to get a better deal.
What made it fall? Out of sight, out of mind for one reason. When it's so rare that it hardly ever changes hands it's hard to keep increasing in value. Plus the fact that we might never know the full story behind the book and if it was really distributed or not (as we've seen with NY World's Fair, being printed first doesn't guarantee that it's more desirable than the next best alternative). And while there's no question it is the first Sub-mariner, it's still not Marvel #1.
Certainly doesn't mean that it's not a darn good book - it is. Just that after years of reflection the market has decided it ultimately isn't as valuable as Action #1 or Marvel #1.