You would then just create the same problem we have now between ZL2 to ZL1 and shift it to ZL1 to ZPL.
I can't speak for other clubs but if we were to lose our 3rd grade it would hurt us big time, as many of our "clubmen" play there. This year we have had 65+ people show up to training, fortunately we have been able to form an All age side which is essentially our 4th grade.
I understand every club faces different factors into finding enough players due to various reasons but getting rid of 3rd grade in ZL1 would only be detrimental to the Zone Leagues. If anything it would be best to maybe try to slowy integrate 3rd into ZL2 over a few years.