Have you failed to notice the statement about 9-car trains, where the current 1973 Tube Stock maxes-out platform lengths with only 6 cars?
20 axles* is not a vast reduction from 24 (and for all we know the total train weight might well be lower too), but articulation is far far gentler when trying to avoid track wear on corners.
*22 axles if these are configured as half-train units, despite having no cabs in the middle, just like S Stock
Some articulated trains have a pretty aggressive behaviour in curves, though.
I'm a simple man, if Siemens are reading this, all I ask is for an interior *not* derived from the Class 700!
Well you can see the interior in the pictures