Re: why doesn't seymour use other magets?
I really think most of what 99% of guitarist want to hear with a humbucker can be acomplished with the A2, A5 and Ceramic. The A8 is also cool, which is why it's becoming more popular - but IMO it's less versatile than the other three. Customers want the sound and feel that they recognize.
I don't agree that 99% of guitarists needs can be accomplished with A2, A5, and ceramic magnets. There are
many HB's that turn out not to have the desired EQ's for individual guitars and woods (and tastes). Many of us have bough PU after PU trying to get the tones we want; sometimes we find it, sometimes we just settle for something less than ideal. As we have learned on this forum, magnets can solve most (but not all) of those issues.
The problem is the average player has no idea what's available magnet-wise, or that he can even change magents at all. There
is a valid use for having a variety of replacement magents available; the members here have proven that. But without the communication to the average player, the demand's not there, so he'll buy and sell PU's in frustration instead of tweaking them. Look at how many JB's are on the used market (I sold a few myself); but with an A8 or A2, the JB turns into a
very different PU. Or how many guys have sold ther C5's for being too bright & scopped, when a C8 would have given them what they wanted.
Versaility is also a factor. While A5's & ceramics are the most versatile, with A2's close behind, A8's don't really work in neck PU's, and A3's are a little bright & weak for the bridge. Manufacturers like magnets that work in both the bridge & neck slots. That still doesn't explain the scarcity of A4's, which some of us find very useful, for when an A5 is too bright and an A2 is too dark. All of the magnets have valid uses in certain situations, and can mean the difference between keeping a PU or selling.
I agree with the comment "Customers want the sound & feel they recognize" but not in the way you're saying that it justifies less magnet choices. Many times to get 'the sound & feel we recognize" in our guitars, thru our amps, we need to use different magnets to acheive that. If you want to sound like a guy that used a 1960's Marshall Plexi, and you're using a different amp & guitar than he did, just buying the same PU he used
probably won't get you sounding just like him. Wood is far too variable for that to happen. That's where magnets can shift the EQ to where you want it to be.
The need for more magnets is real. The knowledge of what can be done with them is privy to few of us, so the demand is small. One day that may change.