Ah, I'm sure that there are more on the way from ESU. It wouldn't have made sense for MM to order millions of chips in one go, then have that money tied up in something sitting on the shelves for a few years.
These chips probably would suit the 141's / 181's as well, so there are a lot more locos out there in need of chips, aside from the 121's.
Reading some of the posts on here, in particular from @murphaph
, DCC is a different animal nowadays. It's gone from being Windows 3.1 running on DOS 6 to being its own OS, so there has to be far greater demand than there was in the old days.