Sharing an opinion is hardly pointless, but allow me to respond to your points.
1) Nostalgia isn't part of the equation, except perhaps for the naming of Buick as a core brand. It is only core in the sense of its historical value and quality ratings, certainly not its contribution to GMNA's bottom line.
2) What plethora? Three doesn't make a plethora. The three are Chevy, Cadillac, and BPG and all are required for GM to continue as a viable player in North America. I say this because there is no way that GM can retain a significant portion of the BPG sales with only the weakest link: Buick. Without those sales, GM has no chance of generating enough income to ever right this ship.
A GM composed of only Chevy, Cadillac, and Buick would be lucky to maintain a double digit marketshare.
That's a one-way trip to nowhere when you have the sort of obligations GM has.
In short, GM needs the volume currently provided by GMC and Pontiac to survive. That volume will not magically move to Chevy and Cadillac, and certainly not to a nearly invisible Buick.
Could Buick be remade into a true player?
Yes, of course it could - over time
In the meantime, It needs the showroom draw of GMC and Pontiac just to stay in the game.
BPG has major potential as a unit, but each component needs the other two to make it work.
And GM needs BPG to preserve an adequate customer base to build from.
The quick amputation of brands such as Saturn, Hummer, and Saab are beneficial in obvious ways.
But, cutting deeper in a rash fashion would be a mortal wound. If you slowly ease one of the BPG group out, it could be done. But that would take a great deal of time to do without causing GM to bleed to death. Killing the channel and saving Buick as a scrap would be suicide.