Given there is little overlap in services between county councils and districts there isn't a lot of scope for de-duplication other than in areas such as HR administration, and most councils are already cut to the bone on staffing, ( despite what some media sources might tell you ). Also, the transition costs will be way higher than they will admit to, as they were in the 1990s version of LGR.
The big mistake in this current round, imo, is that the new authorities are being stitched together along the boundaries of the existing districts, rather than looking to create new more logical units drawn on effectively a blank canvass. In our case, Dolphinholme in the Forest of Bowland, is split between Wyre and Lancaster councils because the river running through it is the Borough boundary. The new authority layout merges Wyre with Blackpool, and will inevitably tend to focus things westwards towards Blackpool, which has very little in common with the areas towards the M6 and further east. It would make much more sense to make a new boundary west of the A6 and put Doplhinholne and it's surroundings totally within Lancaster, which is the nearest major centre. But then again, this was never about making sense.
It's not as if the new authorities would even match the Parliamentary boundaries. There is no logic to the proposal whatsoever.