I know it is functionally immaterial, but boy howdy do I ever wish we'd chosen a better convention for how to refer to the relationship between these system components.
There's nothing wrong with Master/slave. We also read man pages, shove male plugs into female ports, fork and kill children, and use dozens of other odd words to describe perfectly harmless things.
Because there is no reason to screw with common terminology to begin with.
Neither Master nor Slave is a derogatory word. And using the concept of slavery to describe a relationship between inanimate objects in a technical context is not an endorsement of such practices between humans.
I try to use 'primary' and 'replica'; but depending on who I'm talking to I find myself slipping back to the Master/Slave terminology.
And to the OP, replication is a complex process fraught with security risks. Take about 10 minutes thinking about just roles and authentication in the context of your proposed syntax and what you would need to set up ahead of time to make it work.
The current method of starting with a pg_basebackup on the replica is nearly as simple. Even if it does require that you do some minor configuration on the master. It's not nearly open heart surgery.
Someone who has a problem with master/slave terminology probably can't think abstractly (in the same sense as someone who fails the all green birds have two heads test).
I know it is functionally immaterial, but boy howdy do I ever wish we'd chosen a better convention for how to refer to the relationship between these system components.