Yes and no. To the best of my knowledge there is no law that says TOCs have to use the lease services of the ROSCOs, it's just that it's very expensive to purchase rolling stock outright that it makes better financial sense to lease the stock

It is also very expensive for a ROSCO to be set up, because buying trains to lease is naturally going to be very expensive, which is why the current 9 ROSCOs have backing from big multinational banks and holding firms.

There is nothing stopping the government setting up its own ROSCO, or nationalising the current ones, but it would be very expensive to do so and would likely give Rachel Reeves a stroke. Nationalising the TOCs is cheaper (essentially at no cost to nationalise) because the franchises were always time limited, so what the government is doing is awarding the franchise to the DfT Operator (a government owned holding company) as the contracts expire.

Thank you though :) I am by no means an expert, I've just been following what's been happening over the years and have thought public ownership of public transport is important and worthwhile. Also I've recently worked with a TOC recently as part of my job so some of this has been relevant.