Economy of scale matters, so does practicality. Which one is generally lasting longer per number of charges and what's the long term viability of both given the time they were build and the available tech at that time? I totally understand the greater availability of sodium vs lithium. However, will it last? Last time I read much about it, reliability was weak, charge capacity over time dropped drastically, and failures were high. (It has been a couple of years, so things may be changing. )
Something new and shiney can be nifty, but past that, what is this? It seems like an expensive hood ornament that will rust in the rain. Lithium is expensive and toxic to mine, but so are all metals to some extent, and this has plenty.
It seems like it's buying something 25% off on a $100 thing that won't last well. Sure, you saved $25 once, but you're buying 3 of them in the same time frame.
Literally, you answered your own question. From the user end, unsupported file types of any frequently shared format are garbage. No one cares on the user end about server space. They care about sharing a funny image. They don't care about 2 extra ms of load speed. They want shit to just work.
It's the same reason Open Office sucks. You can't rely on it to just work. As much as dev's hate it (myself too), reliability is king. Webp fails this measure, badly.