Is there an argument to say the ROSCO's abuse the market?
There was a long-running and testy argument between the DfT and the CMA (Competition and Markets Authority) about the ex-BR rolling stock market.
DfT accused the Roscos of profiteering, but the CMA said it was the DfT's fault for fixing the market (reviewing all TOC procurement and directing what to buy).
Things are slightly more fluid since then (2010-ish), with more Roscos and a lighter hand from the DfT (except for IEP, Thameslink and now HS2).
Off-lease stock is a sign of the market working, with the Rosco losing revenue if it can't lease the stock.
So the idle fleets of class 379 and soon 350/2 and other cascaded EMUs means the Roscos have to work harder for their money.
Also the remaining ex-BR fleets will be razor blades in the next decade so the historic issues will disappear.
On ownership, the government doesn't own commercial ships, buses or aircraft, so why should it own trains?
Leasing is common throughout the transport industry where the operator has better uses for the capital, or can't raise it.
There is even more "scoreboard pressure" on the Treasury at the level of government debt.
Labour won't have a magic money tree any more than the Tories.