I have thought the same for years. I don't understand why the facing crossover on the Three Bridges line was the priority. It is used by one train per day. I don't know what problem it is trying to solve. I have never been delayed outside Horsham waiting for platform 3 or 4, even when down Dorking trains had to use platform 3 and then shunt. Even if one platform were blocked by a total failure, the timetable could still function using the other platform with minor delays.There is a rather newly laid (?), seemingly unused siding which goes from P1 parallel to the line to Warnham connecting to the headhunt for the Up sidings at Horsham. I have wondered whether they would electrify and signal it for use to access the Dorking line, to avoid any conflicts with the mainline.
On the other hand, Dorking trains do get tangled up with up trains from the Arun valley. The up trains have a sub optimal departure time from Horsham to avoid a conflict. The proposal you mention would be used by 17 down trains and 15 up trains on weekdays.