@Ollie there are plenty of ESP8266 boards that you can buy, I'm not sure that there's much to be gained in having an espruino specific one.
There's also some advantages in having a chip with more functionality, hardware spi/i2c etc, multiple ADCs, more I/O, etc.
Not to mention that if @Gordon is selling a board, I'm sure he's going to be doing something he can be in control of the quality of. He wrote Espruino for STM32s and I imagine it'd take plenty of effort to keep up the stability and quality of the official builds. The ESP8266 firmware is going great (I'm planning on using a couple of ESP8266 boards at home) but it's not feature complete yet and very much depends on the contributions of other people to keep up with bugs and features. I think that Gordon's set a high standard for the official boards and their firmware, it'd be difficult for him to maintain it for a completely different chipset.
I'm too new at this but my immediate thought was why not use ESP8266 as the host for Espruino and have the board be the shim - with the USB and extra storage et al. I don't really understand why it would use two chips, which are both capable of running Espruino.
I've missed stuff, I'm sure I must have. My apologies in advance.