Joseph your stats assume that everyone who is reform or conservative today was since their inception. Many people over 40 who are secular today have/had observant grandparents or at least great grandparents. The amount of time for non legit converts to assimilate into a family trees is not as long as you claim.
Over 90% of the BTs I know had no questions about their Jewish status and grew up conservative or reform. Unless you feel the non real Jews would be less likely to be “chozer beteshuvah”, which one could argue.