But at least if you'd bought the previous low-end model (because of price), you could, after 3 years, upgrade it with more RAM and SSD etc. Now that is not possible.
My question to you is this: If so few people bought a top-end mini, why do Apple offer top-end specs in the 15-inch MacBook Pro line then? (i.e. Quad core CPU).
Surely most users don't need this either (they may want/need the larger screen though.)
I personally don't care too much about the HD options and soldered RAM (I'll just buy the max, which benefits Apple), but the lack of Quad core and Iris Pro makes the 2014 model relatively pointless (compared to a 2011 model!)
At a guess? I'd say because they're pro machines. People who pick them tend to need them to be fast (most "consumers" will pick the MBA or 13" pro). People who tend to pick the 15" will need a fast machine.
Again, this is just my opinion - maybe someone here works at a shop somewhere and knows sales figures - but I'd be very surprised if the top end Mini sales accounted for any more than half a percent.