Why do better MC cartridges cost a fortune?

Yes - very expensive exercise to buy, audition and then sell the ones you didn't like at a discount.

I tend to settle on favourites and hope never to have to replace them.  I am running only two systems now and never want to replace my Koetsu Urushi - nor am I investigating if anything else sounds even better (I'm sur something, somewhere does....).

As for Ferraris, I've owned maybe 75 cars and of those, the only one that ever appreciated significantly was a vintage Lamborghini.  Wish audio gear did as well!

Miura or early Countach?

The biggest problem with Ferrari and the likes is not the cost, you need to know how to drive or you are fool and possibly dead real quick. The second problem is that in many areas there is nowhere to drive such a car, even BMW M3 might be too much.
If I was a very rich man I would do the following. I would get that Continuum table with arm and a few different top phono stages and audition many cartridges at different price levels, including vintage. After that I would post my findings here describing the sound of each in great details. Someone among us could do it.

Whart,

Halcro- have you compared say the Airtight (not even the Supreme, but the original PC-1) to the comparable Lyra (which at the time, was a Titan i, except for the "limited" production Olympus)?

I've had several Lyras including the Helikon, Titan i and Atlas and my friend Richard, had the Olympus for many years on his Caliburn/Cobra.
The Olympus was nice and the Helikon was listenable without the nervous high-frequency prominence and lack of body and soul that shrouded the others.
I have not had first-hand experience of the Airtights except at Audio Shows.

Is your proposition that a cheap MM is going to sound every bit as good as something in this realm?

No.....my proposition is that a great 'cheap' vintage MM cartridge sounds BETTER than most of the current high-end, high-priced MC cartridges. And as good as the very best vintage MCs I've heard.

I have a few 'current' MM cartridges (including the winner of the Fremer Shootout Audio-Technica 150ANV) and can say quite confidently, that they have generally lost the 'magic' of those MMs created in the Golden Age of analogue.
They can be good....just not great. 
The same story with MC cartridges in my experience....😢

Halcro- name a few if you don't mind.
 When I get my vintage tabled system up and running, with a removable headshell arm, I have no reluctance in trying a variety of vintage MMs. It will also relieve me of the need for a step up. 
The Airtight was at one point likened to a London/Decca on steroids. I never had the guts to run a London/Decca, despite the raves about it from some owners. But vintage MM or MC- I'm good with as long as there is no damage to the records. Do you get them re-done by someone- or just try to buy ones that still have life?
thanks,
bill hart