Opposed to this I'm quite happy that it's not aluminum.

Aluminum has the main advantage of being lightweight, however IMO
there are two major disadvantages which I, as the average car owner,
don't really want to take:

.) I've heard repair cost for body work (i.e. the frame) for the A8 in
case of an accident are enormous. You can't have anything "just done"
at the next shop, because Alu welding is not that easy.
Aluminum alloys need to be thermally treated to achieve their optimum
stiffness and strength. So whenever welding is done a post-treatment
of the welding is needed. Also local deformation changes the structure
of the alu and you'd theoretically need to heat treat it.

.) Galvanic corrosion
Aluminium does not work too well with steel. It can be done (and is of
course done quite often), but special methods need to be applied. I.e.
a steel screw turned into an alu thread is almost inevitably going to
freeze (don't now the proper word). The reason is that there are
minimal voltages flowing between alu and steel due to their galvanic
properties. This being accentuated even more by (salt)water and other
corrosive fluids. So properly heli-coils or bronze inserts would have
to be used to lessen that effect. That means that everywhere the steel
meets alu you'd have to implement some kind of protection. It's just
expensive and mistakes can happen. Especially in the shop when doing
repair jobs.

YMMV

Wolfgang

Regards

Wolfgang