Much as the cultural chauvinist in me is enjoying the push for Scouse primacy, it’s worth noting that Liverpool and Manchester rose to prominence at the same time and long had a bitterly symbiotic relationship (until they went and built that bloody canal). A more dominant Liverpool means a less dominant Manchester, but one could never be subsumed by the other.
Not to say there isn’t scope for mega-Liverpool; one that subsumes Warrington and Wigan in the east, stretches to Preston in the north and Chester (and maybe even Wrexham and the industrial towns of Deeside) in the south… there’s a lot of infill to be done to make that happen, but it can happen.
(sorry for the tangent, but I’ve thought a lot about this over the years)