The 737 retirement likely derived from UA's decision to cut back its schedules, and not vice versa. Most people buy cheap seats... with the increasing cost of airfare, coupled with all the additional new fees (including the ones UA and OA will likely invent), flying isn't as attractive anymore (especially in this economy, with everybody battling rising energy prices). Less demand = .... I bet it'll be a very long while before we see schedules restored to "post-9/11" frequency.