why is a 13" tonearm design really superior?

The main reason for the extinction of long and super long tonearms our days is because there are a lot (expensive...) turntables out there which can not mount 13" and 14" tonearms at all.
There are already more than a few which have difficulties accommodating 10" and 12" tonearms (Basis Debut Gold, Linn Sondek, most plinth-type TTs - for instance).
Furthermore - of course - increased length is always a trade-off between rigidity and effective moving mass on one side and minimized tangential error and minimized VTA change during warps and uneven areas on the records surface.

ok that´s a limiting factor of course, understand.
Today a friend of mine showed me a pic of a 24" tonearm.
He listend to it and was overwhelmed. He will find out what manufacturer it is, I will report on it.
Maybe the headshell speciality with very long straight tonearms like the SAEC 8000 is also a kind of issue.

Thuchan, there is another solution, namely a straight-line tracker. There are many issues with it also. I did see a pivoting straight-line tracker at CES, which used a second arm to keep the headshell and cartridge always tangential to the grooves. I don't remember that name, perhaps because the arm cost $15k.

Tbg, I guess, it is the "Thales" Arm. Made in Switzerland.

YES it is