For me the issue is there is clearly enormous investment required to upgrade the infrastructure. It ought to be possible to crack on with this a bit better if centrally funded, and the buck would stop in one place.Though worth noting Dŵr Cymru is a not for profit and things there are not exactly going swimmingly (sorry). Problem is water services have many challenges that are not overcome by state ownership; building new reservoirs is hard - not the actual building, but getting the planning permission in the area that need the water (cf. Abingdon reservoir). Expanding sewage systems to cope just with increased peak rainfalls let alone the changing habits of the population, the growing population, the ageing population is also hard; again not the actual building part but nobody, absolutely nobody wants road closures while sewers are upgraded, while as for finding a location for new sewage treatment works...
Would it necessarily be run super efficiently, probably not. But it is too important an issue to have the current lottery where the service is dependent on who your provider is, and a great deal of money is going into the pockets of shareholders, which could and should be paying for new pipes, reservoirs or whatever.