What's so funny? The Audi A8 is on the MLB platform with the A4
If so many on here are going to simple dismiss the XTS because it share's a few hardpoints (but not wheelbase, roofline, doors, overall length, suspension bits, drivetrain or interior) with the 2012 Malibu... then I'm free to do the same thing to the A8.
MLB is a very loose definition of a platform, I believe, more of a component set. The A4 A5 A6 do not use aluminum space frame construction like the A8. Those cars don't actually share platforms with the A8 in the sense GM or Ford have traditionally defined platforms, or even in what GM calls architectures (even though the GM definition is closer to what the VW Group aimed at with the MLB).
In any case, the point many of us are making (implicitly) is that if XTS and ATS shared a platform no one would complain. But the A8 does not share a platfom with the Passat; it's an Audi (and eventually Bentley) thing. Another important point is that for the money GM is spending changing those Epsilon2 harpoints, etc, perhaps GM would be able to properly reskin a Holden Caprice, put in a Cadillac-level interior and have a proper RWD flagship right now instead of falling into the same "it's coming" talk we've all read for 20 years. The XTS's styling would be perfect for the proportions of LWB Zeta, and to me it's a shame to see that opportunity fly away once again.