The China , India and Pakistan is a good argument. They hold around half of the worlds population so if we go by the rationale that the JW truth has to be preached to everybody, then we are years and years away.
Also point out to them that the reason why they are able to convert anybody with their preaching ( and they are few), is because someone came before them and preached the gospel - converting the entire western hemisphere to christianity. They are just going behind those christian pioneers and giving a different angle on the gospel.
Their preaching work is ineffective, and even the majority of JWs were converted due to informal witness. This type of dragnet type of preaching also has no biblical precence.
Most importantly, the preaching work is really the basis for WT revenue and this is why a huge emphasis is placed on it, despite being unproductive. The fact is that whether JWs are able to make converts or not, the revenue is still there, because publishers pay for the Washtowel Ragazines. This is why the GB has been always so much against using the new techologies to spread the message.
I remember that at some point the Canadian branch was involved in producing movies to spread the message of the JWs beliefs and they were told to stop doing that by the GB, because they wanted to make sure that those type of activites didn't take away from preaching - it didnt' seem to matter which was more effective.