The comparison isn't with the potential new price of the old one, but with the old price of the old one. Say they were paying X per year for #1. Adjusted for inflation, they're going to pay roughly the same X per year for #2, but #2 is more efficient, AND in line with Singapore's positioning as a top airline with a very young fleet.

I also expect that keeping #1 would have a lower lease cost, but a higher maintenance and insurance cost.