Issue is not dev cost, it's existing ecosystem. To drive a smartwatch ("smart" as in running a smartphone OS giving it 2 days battery life tops)[0], you need a full already existing ecosystem, so either iOS or Google/Android. Only Google can get Google to support RISC-V in Google/Android.

[0] there might alteady be less smart watches that are already running RISC-V but we can't know since they are closed.