I think you're missing primary the point of my proposed dual chemistry packs:

LFP and Na-ion are cheaper than NMC or NCA. So the primary benefit isn't cheaper replacement costs (thats just a side benefit). The primary benefit is: lower overall cost of the pack without sacrificing long range.

LFP and Na-ion are lower power density than NMC or NCA. A pure pack of either LFP or Na-ion would give the vehicle a substantially shorter range than a pure NMC or NCA. However a dual chemistry pack would be a cheaper pack as well as still retaining most of longer range.