It seems very complicated. I guess there's lots of people and groups involved, all with their own wants and also an existing infrastructure needing to be adapted to changing terminals as time passes.
In some ways I don't know why all trains and tubes don't just go London to T2&3 to T5. A shuttle service could operate to T4 from T2&3. A same-platform transfer could be set up and if trains and shuttles connected there shouldn't be too much extra time needed to get to T4, on average, owing to more frequent trains.