I thought there were a few shared platforms at VW, not just one:
0. Small platform for non-US cars
1. VW Golf, Jetta + Audi A3
2. VW Passat + Audi A4, A5, A6, A7, A8, Q5 + Porche Macan
3. VW Toureg + Audi Q7 + Porche Cayenne
it was a little hyperbole, but not much. that one platform accounts for what, 90% of audi US sales?
So what's your definition of a luxury car?
More specifically, what features or technology should a car have to classify it as such?
luxury to me means something well out of the reach of ordinary. it's like all these 'luxury' apartment complexes popping up in town: when everything has wood floors, stainless steel appliances, and granite counter tops, it ceases being luxury. viking gas ranges? now there's something different.
ttac had an essay about the lost luxury in the automotive world. the premise went something like this: until the mid 70s a luxury car had the best ride, the best tech, and leather you might actually want in your mahogany paneled study. then GM told its brand veeps that each had to drive cars from their own divisions, rather than all driving cadillacs. so they each turned the division's largest cars into cadillacs. meanwhile, cadillac kept trying to move down market. which car was luxury, the caddy cimmaron or the top of the line bonneville in the next driveway? and if you're a luxury buyer, you don't really want people to mistake your top of the line cadillac for that bonneville anyway.
so you go over to the little mercedes dealer and buy a car from there. it doesn't look like the bonneville, doesn't share a name plate with the cimmaron, and is easily the most expensive car on your block. sure, the ride is stiff, the "leather" is really vinyl, and the interior is spartan at best. but no one else has one.
anyway, that was the gist of the essay.
to me, it has to be big (so you can be driven and do work), it has to have the latest automotive tech (the more unobtanium in it, the better), and it has to be rare. so, MB and BMW do sell luxury cars, but most of their sales aren't.