That's a really good point from rsh. Without actual statistics it's hard to be sure, but there must surely be quite a lot more residents within a short walk (say half a mile) of Rose Hill than there are the same distance from Marple. And as rsh says, with a proper evening (and Sunday) service, and a bigger car park, Rose Hill could also take a lot of pressure off car parking at Marple.
Maybe there was a time, long ago, when coal depots were sited next to stations, but those days have long gone, and Pearsons could operate just as well anywhere in the area. If SMBC and TfGM could get together and look for another site, and then help Pearsons to vacate their present site and move, that could be a big step forward.