You are absolutely correct. Ford has one luxury Brand (Mercury). So does Honda (Acura), Toyota (Lexus), Nissan (Infiniti). Audi isn't a VW brand per se, but there is much engining sharing, so close enough.
What do they know that GM doesn't? That some of your luxury customers traded up from your main line. GM thinks there is room for 2 (Olds was killed years ago).
The tragedy of Saturn is that they used to trade on a reputation and brand that was better than their plastic bodied cars. Then when all hope was lost, they became the way to import great Opel cars into North America, well built, and much better than the Chevy/Pontiac choices in the same size. But it was too late.
The original Hummer was a true military vehicle with a couple of nods to creature comforts. Then the H2 was more civilized. The H3 is really a chevy pickup with a Hummer-lite body. Whats the point in that?
Yes I knew about Saab. When it was an independent firm they were great to drive quirky cars, now they are just bland and dull like everything else. They offer a few unique options but never had enough for a full line. Hence they took a Subaru and made it the Saabaru (92X which was a Surbaru WRX) and the 97 SUV (chevy trailblazer). In Canada they were sold in SSI dealerships (Saturn Saab Isuzu).
While they are cutting, why not get rid of GMC - they have nothing that Chevy doesn't already have and the differences are really minor cosmetics.
Buick is "entry level luxury" versus Cadillac's full luxury, by industry standards.
And for the record, while my dad hasn't owned a non GM car or truck in 50 years, I've never owned one. But my business is dependent on GM and its competitors and so this announcement will mean layoffs and pain for my company. My brother was laid off from Toyota a few months back, but he may get more work in 6 months or so.
James