Law Of Diminishing Returns?

As you can see there are many "opinions" related to your question.  Personally I think the best advise is never pay retail for something you can buy used.  It is true that the latest and greatest is hard to find used but that may not be an issue on certain components as you may not need the latest and greatest.  I have a combination of new and used for instance my Thorens TD-124 was built in 1961 and restored this year, I will never sell it.  My speakers are Klipsch Cornwall IV's that I purchased new and absolutely love (no listening fatigue ever) there are those on this forum who will speak critically of them but they are the perfect speakers "for me".  From someone who has always had issues finding balance, I simply suggest that you enjoy your journey and the love of music.

In the '70s I started with a Kenwood integrated (which I still have) and Large Advents.  In the '90s I moved up to a big Adcom amp and Proac 2.5s.  In the '10s it was McIntosh and used Avalon Eidolons.  And now I have a dedicated room with Esoteric electronics and Audio Physic Avanteras.  Most of these upgrades have made me happy.  And I can't wait for Axpona to begin exploring what my next moves will be.  I guess I'm just a dumb happy guy.

Diminishing returns is a term of art in economics originating with Jeremy Bentham. A classic example is that for most people, the fifth ice cream cone they consume today will have less value than the first or second. it is not linear, and it does not kick in from the first dollar spent. Someone who upgrades from the IEMs that come with a Samsung phone to a $200 pair of IEMs might gain a net higher return from that additional investment. Where diminishing returns begin to be felt is subjective. We might hypothesize that they begin to kick in at the point where someone is 75% satisfied with all parts of her $10K audio system, and is considering an upgrade to gain an extra 20% of satisfaction. Generally speaking, an economist would say that if she has to spend $20K to gain that extra 20% of satisfaction, returns to investment are objectively diminishing. That certainly does not mean, however, that she shouldn't spend the extra money to improve her system. That depends on her bank balance, her subjective valuation of the additional 20% in satisfaction, and her opportunity cost (what else she might do with that amount of money instead of spending it to improve her audio system).  

I had a friend that went through a similar process at great expense.

I took him to a local shop that has a heritage system consisting of original Quad ESL speakers and amplification. The owner played some acoustic music and my friend jusy said "Fzck!" because the reproduction of strings was so amazingly realistic - better than his megabuck system.

Third post nailed it (mulveling). Is that $10k cartridge THAT much better? Only if the rest of your system is.