> Please just let me end rules with a trailing comma instead of a period, I'm begging you.

The reason Prolog doesn't support trailing commas is exactly the same reason JSON doesn't support trailing commas: the language is designed to be parsed by an operator precedence parser, and the comma is an infix operator.

The fact that Prolog is a purely operator precedence syntax, with a dynamic operator table, allows you to build DSLs embedded in Prolog. CLP(FD) uses this to create the #= operator.