"I'm surprised, ... that it took until 2000 for rear disc brakes to even come on a Bonneville"
Why ? "If it works, don't fix it." My Reatta has rear disk brakes and they are generally accepted as a pain - they rattle and the parking brake adjuster often seizes up.
In a nose-heavy car (and the GM fwd cars are), the rear brakes generally do not do very much and the servo action of a drum brake (which disks do not have) makes the parking brakes much easier to use.
I doubt that anyone is going to mistake a Bonneville for a road racing car or make repeated "minimum distance" stops from over 100 (mph not kph). As a consequence, rear disk brakes, like a spoiler (trunk handle) are for cosmetic purposes only.
In other words, from an engineering standpoint, rear drums have a number of advantages over disks and provide "enough" braking for our use. Yes, it does take somewhat longer and a bit more skill to change the shoes.
Think I saw once a fake rotor that can be sandwiched between the rear wheel and the brake drum if you really care.
ps one of my first cars, a 1959 XK-150S Jaguar, had four wheel disks and have had a number of similar cars (Reatta and Fiero are the most recent). Reatta also has 16" wheels like the XK so have come full circle. Bonneville is fine as is (and it doesn't take several pumps of the parking brake pedal to make it work).