Are interconnect cables worth $15,000?

The cables you are refering to are actually pretty cheap cables. It is the cost of the termination that gets you. You can buy interconnects for $15,000 for 1 meter terminated with either RCA or XLR and then get another 1/2 meter for only $2,400 more. Based on that scenario, the termination cost and connectors cost you $10,200. By comparison, the cabling is dirt cheap and relatively quite a bargain.

I believe that 99% of all systems will benefit more from a $15,000 component upgrade than a $15,000 interconnect.

Or you can add room acoustic treatment or a better house with a more suitable room to that list - I agree wholeheartedly.

IMHO $15,000 cables are nearly the most expensive inflexible (poor value) tone control that money can buy but I respect those with $15,000 Cables who claim to hear fantastic differences....so to each his own...audio is an emotional/perceptual thing; to some extent high prices provide comfort (compared to radio shack quality) that no stone is left unturned...no compromises allowed.

Who is asking,
is what I want to know.

Everything is worth only what you are willing to pay.

No. Not in my universe.