I agree with your question about why it took all the centuries from Jesus til now to get 144,000. It does seem a little far-fetched and presumptuous to assume this number was only filled in the late nineteenth and twentieth century. This implies that nobody qualified as being "spiritual" before this time.
Of course, this bias is in favor of modern day JWs. Why would Jehovah/Jesus have a partiality to any particular person in any particular century?
Explain to me who goes to heaven and those who will live on earth