I have kinda stayed away from this thread, but as I think it through, hopefully my input will hold some merit....
"Why are so many companies like harbeth making old speakers designs and charging thousends?" Thousands....
Really, in the big picture, Every speaker design can be called Old.
What changes speaker design is that a company or designer will like the look, spec, sound of a new driver and develop a new speaker around that driver or drivers...
In the end, most designer prefer a sound of a certain type of crossover slope and will normally gravitate to using those slopes when possible.
A 6/12 slope has a certain sound, all 6db slopes another, 12/18 or all 24 db slopes, etc, etc, etc. Each type of crossover has a flavor that each person prefers.
Harbeth in particular has their own midrange driver, Unless they contract a new part or parts, I have to believe that they will continue to use the parts that they feel is their best foot forward. Maybe each company needs to try different parts, as in a ribbon or a horn load, or exotic material drivers, but once you get a potion for success and you make good money doing it, it gets hard to rock the boat.
I know there is a huge following out there for all 6db per octave speakers, I personally like Odd order slopes, properly aligned and phase compensated.
So in the end, true NEW parts being introduced doesn't happen everyday and right off the top of my head, I can think of maybe 6 or 8 or so different crossover topologies. The 2 that I use most are most likely butterworth or Linkwitz Riley. New crossover technology hasn't happened in decades.