Apple doesn't need to be successful in order for these computers to remain valuable. They have a very important place in the history of computing regardless of Apple's continued success.
An important place in history has little to do with monetary value. Supply and demand has everything to do with it.
And if fewer collectors care about Apple, the monetary value of Apple collectibles is likely to go down.
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Apple's cache is fading? What cache are you talking about? The cache in my MBP seems to be working just fine, not "fading" at all if you explain what it would mean for a cache to be "fading".
The cache I'm talking about is mindshare, as indicated by market share. Fewer people think well of Apple now as compared to the recent past.