[bluGill]

There is a lot of off the shelf 12V equipment you can buy. Plus even more that is sitting in garages ready to be installed in the next vehicle. Cars are manufactured in enough quantity that it would only cost $0.01 per vehicle to design it (plus parts costs which are probably the same), but that is still a few million to the bottom line if they use the same 12 volt radio. Add to that that ICE cars everywhere have 12 volt starters, and you can buy 12 volt jump start kits: when (not if!) a battery fails to start the ICE you better be able to jump start it from a 12 volt battery - this is a safety issue.

Tesla doesn't have ICEs, so the safety concerns are lost on them. Thus all 48 volt makes some sense. They still need something for all the accessories people have.