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Except that Pontiac has never been a luxury brand.

The Chevrolet Malibu was the final nail in the Saturn coffin.
The Malibu proves that Saturn is all wrong.
Everything GM wanted to do with Saturn can be done with Chevrolet.

GM doesn't need to cut brands or lose products either.

For example -
Let each independent dealer know that Pontiac will have two cars, take your pick.
Saturn will only have a mid-size car, a sporty roadster and a smaller minivan.
GMC produces one fullsize truck and a mid-sized SUV.
Buick produces only a luxury full-size car and full-size SUV.

Regardless of the example, split it in the way that makes most sense -
the key here is that between the four or five lesser nameplates,
you build a single cohesive non-overlapping full-line of vehicles.

And your dealers need to consolidate if they want a full range of product.
Overall you haven't lost any brands.
Overall, you haven't lost sales because all you've done is cut out the duplication.