From this gent.
“It would bring in new management and prevent the nonsensical separation of post offices from mail.”
This completely contradicts the analogy with Network Rail, where the “natural monopoly” element (the rails) are operated as a not-for-profit, but the use of them is open to competition. You could even argue the same thing about the roads, which are operated by the state with people able to run competing buses (with the exception of London), taxis, haulage, etc. on them.
If someone was trying to replicate the Network Rail model, they’d completely separate the post offices and the mail, operate the mail (which is the element with natural monopoly characteristics) as a not for profit and leave the supply of post offices to the market.