@pantonyp: I hope you know by now, that there is an automatic replacement tool for each line, where you can set the vehicles to be automatically replaced with a chosen vehicle when they stop at a station/bus/truck stop after a predefined time (25, 50, 100, 200…% lifetime of the vehicle). True, I still would also add a 0% in cases where you receive a new model and you just replaced the vehicle on the line with some old model, but that is a small annoyance.
Also if there was a possibility where the replacement would be automatically setup by default and you would not have to set it for each line, that would be great, but it is still a great help and if you do it for every new line, and not as me only when the old vehicles start screaming, it is easy.