As shown in Vignesh's email [1] (point 4), there could be multiple additional parameters required for the first option suggested by you, which will make it longer. Additionally, there are some other benefits of having the second option (pg_createsubscriber --create-standby), like better cleanup of contents during failures and better progress reporting. Are you still against adding such an option?
None of those benefits convince me that "let's write a shell script in C and put it under an annual feature release policy" is the way to go here.
Let's make something like this available to the community, say on the Wiki, and make it work in PostgreSQL 18 so they can benefit from it today, and readily use it as a basis to tweak things for their own unique circumstances.
