It does almost by definition. Sewers are a natural monopoly, it's physically infeasible to switch providers.
Yeah I know this model but here the one infrastructure is the service.
It's really hard to see how this works with water. Upstream of your taps is an essentially fixed set of water collecting infrastructure. Downstream of your waste pipe is a natural monopoly sewer system which collects waste towards a single geographically determined water treatment plant. What is there for a competitive operator to change?
The honest answer is that water privatisation was the product of a very specific brand of 80s-00s magical thinking that believed the private sector was necessarily more efficient even without a plausible model of how the efficiencies would happen. There's a reason water and rail were two of the last privatisations, long after all the things like British Airways where there were reasonable grounds to believe the private sector was a better home.
Many fundamental components of society aren’t amenable to market based competition. The public sector can be inefficient, but in many cases the alternative is a form of economic tyranny that is far far worse. In the UK, most of our key infrastructure has been privatised and it has been a disaster for our economy and for the population. One of the likely causes of the UK’s productivity crisis is the lack of investment in infrastructure caused by privatisation, for example.
This is all rather self-evident but I’m so surprised how very intelligent, highly educated people often are unable to see that government is sometimes superior to the private sector. It seems their judgements are clouded by dogmatic ideology, but because that ideology is capitalism they are unaware that their dogmatism is no different from the others that they regularly mock and criticise.