I totally agree. It is not only a need, it is often literally game braking. More then once I had a crash to desktop when I tried to replace a deleted station by a bigger one. So, a simple upgrade button with platform and track expansion is very welcome. (And I like to see also other upgrade tot the station to convince people to use the train, like a parking lot,…)
However, if the space needed for the expansion of the station is taken by roads and all kinds of buildings, it is logical that the player needs to clear the planned expansion space, as it is in the real world: buy and bulldoze the neighbouring houses, and expand the station.
I can usually replace a station with the same platform length on the exact same spot. Sometimes I have to bulldoze some surrounding roads and a few houses which are near the station but if I just connect it to the peace of road it was originally connected to I don’t even have to replace the connecting railroads.
But I agree though, it should be possible to just add/extend platforms without demolishing the entire building…