I have mixed feelings. On paper I agree with you, people should start with POSIX shell. In practice I'm not sure how relevant that is anymore. I'm not really convinced bash should be the default thing people learn, but I think there's a decent argument that people should just start off with the shell they're going to actually use. You should, however, be aware that there is a distinction, and if you're learning/writing bash and not POSIX shell you should specify /bin/bash not /bin/sh. But you don't necessarily need to know all the nuances of how bash differs unless you have a need to write POSIX compliant shell.