But is the sliding head really necessary? They justify it by the
arrangement of bearings, typically two being used on the spindle end,
one on the other end. Why not fix the headstock and put two bearings at
each end and create a true outboard lathe?

There are design considerations that come into play. A common lathe
headstock will have a bearing set with high radial and axial thrust
qualities nearest the spindle at the inboard end. The bearing at the
outboard end is not so heavy duty and its job is to keep the shaft inline
and support the belt loads, it carries none of the turning load. While the
shaft is firmly attached to the inboard bearing inner races, the shaft is
allowed to float in the outboard bearing. This prevents heat form applying a
damaging force to the bearings as the shaft lengthens.

Making both ends able to support turning is no easy task. It requires a lot
more engineering and a more complicated assemply, which also means a more
complicated belt changing, and increased cost.

Finally, in the Robust headstock, even if the outboard bearing was
increased, you wouldn't be able to turn very large bowls since the motor
would interfere. And, you'd need some sort of outboard tool post
arrangement. I have found that all outboard tool post to be lacking in
rigidity. Plus, you'd loose your handwheel, and finally, you'd need a new
set of faceplates with LH threads.

Dan