I feel your pain. I had a beautiful '93 Riviera that was perfect, inside and outside; except for the underside. The whole underbelly was rotten. Brake pipes were replaced, air ride sensors dangled from their wires, as the mounts had rotted away, the suspension looked like it came off the Titanic.. Scrapping it was done to put it out of its misery.
I've got a '07 Escalade that too has major rust issues in the engine bay and underneath. Trans cooler pipes rotten, exhaust manifolds rotten to the point of leaking, power steering pump rotten.
Perfect body.
Back in the day, rusted bodies were a big issue. Somehow they (GM) got that fixed with better steel coatings, but they didn't follow thru on the underbelly.