Still, needs doing, one way or the other. The plan, way back when, was that the local services would be sent into a tunnel at Broad Green and become part of Merseyrail. Thus, it was presumed a near certainty that four overground tracks would be never needed again.
As well as equipment on the trackbed, the supports of the Queen's Drive flyover currently leave no space for four-tracking. However, there are plans to demolish this flyover, so there will be an opportunity to remove this constraint. The other issue is Wavertree Station, which pretty much would need rebuilding.
These complications may mean revisiting the tunnel option but if the locals go underground, Wavertree will need the long distance services to call at it. It currently has 4tph and I don't think you could get away with dramatically reducing that. Otherwise, you'd have to consider a new underground station to replace the existing one, or put the long distance services in a tunnel instead.