What you fail to take into consideration is that 'baptism is a requirement for Christian discipleship' and that baptism is only valid if the person has accurate knowledge of god and his purpose. So anyone baptized into another religion has an invalid baptism. This was stated in a recent Watchtower study.
I'm not sure the exact year the WT took that stance of rejecting baptisms of other Christian sects but it was taught for a number of years that a fresh baptism was not necessary.
Considering the above, most of the early "anointed" would be in the position of not having a valid baptism and thus not actually anointed (oops!) I would also argue that a) since the JW versions of accurate knowledge changes every year, all prior JW baptisms are invalid and that b) since JWs always have inaccurate knowledge anyways, with their cult twisting of scriptures, that all of their baptisms are invalid according to their criteria.
So no worries, plenty of slots open then.. Assuming the millions of Christians coming before them didn't take all the slots already.
JW theology is so full of holes and discontinuity that it would be laughable if they didn't have such bad effects on people.
Incidentally, when I started heeding the admonition to read the bible daily, the 144k deal was one of the first obvious flaws that I noticed.