When I think about the 144,000 being derived from 6000 years it's telling me the completion, consummation and pinnacle is reached with the number of 144,000; that is to say, once God finds His 144,000 virgin firstfruits believers (Rev. 14.1-5) then the first rapture takes place according to readiness (Rev. 3.10, 12.5 at 7.9 "before the throne"). That is not to say there are only 144,000 saved.