Apple cares about selling the most higher margin/expensive products possible. THAT is the reason they do not produce a quality Mac Mini product.
It doesn't get any simpler than that - they don't want to lose the sale of a SINGLE higher margin/expensive product.
Well if all the units they are selling today were manufactured years ago and are being drawn down from stock stored in a warehouse somewhere, that would support Apple not upgrading it since they need to sell all that remaining stock and if people are willing to pay $499 and up for it, why put it on Clearance and discount it to move it faster?
If, on the other hand, they are still being manufactured today (just in very low volumes) to meet sales demand, one would expect they have a pretty nice margin (at least on a percentage basis) considering how cheap the components should be due to age.
Regardless, Apple would be under no obligation to keep the $499 entry price point for a new "quality" Mac Mini (either once they sell-through existing stock or stop production of the current model). If the base price needed to increase to $599, $699 or even $799 to maintain their desired margin, they would do so.