Remember that railways only go more or less in a straight line. bus routes can get you to new places straight away while it might take 20 years to get a train in place. For example Tallaght shopping centre or Dublin airport. The day they opened a bus route could be put in place from the city centre, but it can take decades to get a train line there. Who's to say in 20 years time the places people want to go won't have a train line connected to them?
Eventually everywhere will be close to a QBC and/or train and everything will be just one QBC/Luas/Metro/Dart change from your home.