I agree with the fact that just taking the first platform that is available is not realistic. However, I do not think the arguments to not have it work out: in the real world, trains do not only have a fixed platform, but also a fixed time to leave. Especially at bigger stations, the platforms are assigned in such a way that there never have to be two trains at the same time. In Train Fever however, there is no such thing as a timetable, so it is entirely possible that two trains of different lines arrive consecutively at one time, and simultaneously another time. In the second case, it would be nice to have a backup platform. Because in real life, when a train is delayed, aren’t trains rerouted to another platform when there is no space?
I do think adding dynamics would be a good thing, but I think it is more important to be able to assign platforms to a line, because if there is a station with three platforms, I would like to use two platforms for lines going through, and the other platform for a terminating line.