Cost/gb doesn't matter if you're not using the excess space, so that's not really the right way to look at it. Suppose a 64gb cart costs $5, 32gb costs $4, and 16gb costs $3. If a game only needs 16gb, they'd save $2, regardless of the /gb.

And of course, Switch 1 carts are still being manufactured in those sizes. Not like they're halting all production of S1 titles overnight.