There wasn't a "theory" because this wasn't some garbage that aloof academics cooked up.
At the time thins were privatized (50s through 80s) the misalignment of incentives was plain as day obvious fact. People looked at <shuffles cards> New York City, and said "do not want" and they attempted to break the feedback loop between public agencies and the parties they were making work for and tried to resolve it by putting more of the decisions of what work needed to be done under the umbrella of the agencies doing it. With proper competition, this can work. But people like you have spent the last 70yr erecting barriers to competition and so in an environment where things are only ever getting bid on by the same few players the costs rise and the values go down.
The sane way to privatize a transit system isn't to give the private company ownership over the system, it's to have the government own all of the plant and equipment and pay various private companies to supply or operate pieces of it. Then they're not deciding what fares are (the thing with no competition), they're deciding how much they bid to provide rolling stock or train conductors and then the government chooses the company with the most attractive bid for each thing it needs to buy in any given year.
The government in turn sets the fares by amortizing the total cost of the system over the number of riders modulo any taxpayer subsidies it intends to provide.
The half-nationalised half-privatised system you're suggesting isn't that far off the UK's approach, which results in almost zero maintenance, no on-platform support for disabled people and fares so high that people fly to other places in the UK via europe because it's cheaper than taking the train.
At the time thins were privatized (50s through 80s) the misalignment of incentives was plain as day obvious fact. People looked at <shuffles cards> New York City, and said "do not want" and they attempted to break the feedback loop between public agencies and the parties they were making work for and tried to resolve it by putting more of the decisions of what work needed to be done under the umbrella of the agencies doing it. With proper competition, this can work. But people like you have spent the last 70yr erecting barriers to competition and so in an environment where things are only ever getting bid on by the same few players the costs rise and the values go down.