FWIW, there are WORM microsd cards available but it looks like they still use flash under the hood.
Because of the Internet age there probably is not much place for write once media anyway, even it would be somewhat cheaper. But maybe for specialized applications or if it would be much much cheaper per GB.
Because the older Flash aren't as stable when miniaturized as you'd expect. Current flash is a direct descendant of these, they only are more stable because the cells are much chunkier and thus with lower leakage.
That being said, it's not clear how innovation has occurred in this direction in the storage space.