I think the point in the question is that the displaced threshold was there for obstacles that are no longer present. So, what is the continuing purpose of the displaced threshold if the limiting item is no longer applicable?
Following on from that are further questions:
To whom do the threshold rules apply, and when? and,
Is it ever permissable to define your own threshold if the operating surface is suitable before the 'irrelevent' threshold?
What approach gradient is the threshold (displaced or otherwise) predicated on? and finally,
What approach gradient are you required to use for your operation?
Some of theshort runways I operated into on public transport flights in the UK were never affected by a displaced threshold - there was not enough room in 381m to allow for one! Even with obstacles on short final that were best suited to a techniqe of 'dropping on' for the last 50' or so instead of maintaining a constant gradient.