This just makes the case for supporting it even further, because then they'd be supporting multiple years worth of hardware with just the effort for one uarch

Nothing like having a lottery of 2-6 years of support for your hardware to make your customers confident they are getting value out of the products they are buying.

The manufacturer can smugly proclaim they offered six years of support for a product that was on the shelf four years into the driver's lifecycle.

Speaking from having run tens of thousands of both 580 and 480, they weren't just rebranded. Maybe on paper they seemed similar, but they didn't run similar.