The problem (at least here in the U.S.) is that the JW's have been hauled into court more than once and charged with violating the laws governing child labor by having children go door to door. The defense has always been that field service is entirely voluntary and is not a precondition for association/membership in the congregation.
We all know that those who don't regularly go door to door are considered weak and poor association. We also know that this is actively encouraged from the platform. But to codify this attitude with a formal change of status for inactive or irregular publishers would yank the rug out from under their defense against child labor
There are ways around this of course. They could quit baptizing anyone under 18 and prohibit anyone under 14 from going door to door. I would be surprised though.