The biggest problem with parallel is not the implementation but the command line options specification. The shell has no generic way of defining multiple stream

The biggest problem with parallel is not the implementation but the command line options specification. The shell has no generic way of defining multiple streams feeding multiple placeholders, so parallel ends up using its own syntax which has to be different in order not to interfere with the base shell syntax. If the same functionality was integrated directly into the shell, it could be less complicated and more generic.