Prices per GB of RAM are starting to plateau, or at least not fall as quickly:

https://aiimpacts.org/trends-in-dram-price-per-gigabyte/

DRAM density and cost isn't improving like it used to.

Also memory efficiency is about more than just total DRAM usage; bus speeds haven't kept pace with CPU speeds for a long time now. The more of the program we keep close to the CPU -- in cache -- the happier we are.