We ride the past...

Notice that it's on a road bike? I wonder how it would do on a mountain bike once it's splashed with mud, dirt and rocks to jam up all of the little bearings. Honestly I like the look of it and the simplicity but I think I could break it with just basic single track riding.

Definately not retro, it's vintage, the shaft drive idea has been around since the 18th centuary:



https://www.coopertechnica.com/1899-Col ... icycle.php

chain is best. No surprise

Indeed, but not the same. Move the shaft forwards and backwards via a shifter of some sort and you have a 10 speed gear system.
Perfect, no, but I like their outside the box thinking.

Not just forwards/backwards but also inwards-outwards to mesh with the cassette. All the drive forces will try to push the shaft out of alignment, so the shifting mechanism will have to be very strongly constructed.

Catch your leg on that!

There was a reason why the idea stayed in the 19th centuary.