The other side to that is that a 4-car train every 15 minutes is a lot more attractive to most people than an 8-car train every half an hour, so going to fewer, longer, trains, would undoubtedly cause the loss of some passengers. And I'm dubious how much loss of reliability is actually caused by the high frequencies causing knock on effects. Sure, there's probably a fair amount of trains being delayed 5 minutes or so because of waiting for other trains/lack of capacity, but 5-10 minutes isn't really going to put many people off travelling. I'm fairly sure the really serious problems (when you get lots of cancelled trains, or trains being an hour late, etc.) almost invariably would have other causes (industrial action, problems on the line, those badly planned timetable changes, etc.) and so wouldn't be helped much by running fewer trains.