Because you aren’t the first one here. Countless lines of software exist which assumes posix. People learned how to use computers and know the grep commands. There is documentation for posix in places where it needs to be. It handles important edge cases which were needed by some groups and that was formalized into a standards requirement.

So changing to the new thing invalidates all that existing value.

That’s not argument to never do anything new, but it’s an argument why for your UNIX-like OS should ship the standard boring thing instead of the shiny new thing in the base install.