I think in some way it's a marketing plan to show that despite Richie departure they are still present in the music industry. I also think they want to reach more fans as they can until the they retire ( i don't think or say it's their last tour but...). At this point it dosen't really matter if they have a new album or songs, since they don't come often their the fans will want the olds songs or big hits anyway. Finally They will be their probably for one month so it won't be a hard and long tour.