Indeed, which is why it should be an option.

OTOH, it shouldn't be significantly more expensive. It should be ~9/8 the cost of regular memory. It's just one extra chip for every 8. Nothing more.

in-band ECC is also a thing. In that scenario, you give up some capacity for the ECC bits but stay with the same DRAM config as before.

(in-band ECC is present on Elkhart Lake Atoms and on Tegra Xavier for example)

Actually less, because you only need the additional memory chip and associated trace layouting, not any additional PCB manufacturing cost (beyond miniscule yield impact of the additional traces) and no significant added distribution cost (packaging, shipping weight, etc.).