I disagree, the ROSCO's need belting with a stick, without the DfT banning stock from the network, nothing will ever change, we will still be riding on pacers in 2040 if the ROSCO's had their way.
That's not how the free market works, and arbitrary banning of stock might even be illegal under the various EU open market laws.
ROSCOs have zero obligation to spend money for the sake of it. They need to either be offered an incentive to do so, or be confident of a gap in the market. If you don't like that, you need a nationalised industry, not a privatised industry. DfT get to set the franchise requirements, parliament get to legislate stuff like accessibility, ROSCOs and TOCs get to choose how they meet the requirements and legislation. ROSCOs are quite entitled to respond to any attempts at arbitrary bullying in a negative fashion, if they are the ones being asked to spend money and take risk. They also have an obligation to their shareholders to maximise the value of existing assets by milking as much profit from them as possible.
ROSCOs can keep stock running for as long as they like, and the model we have requires that they be allowed to make their own choices about when it's no longer reasonable to keep maintaining and upgrading existing stock. If you want them to order new stock before they otherwise would order it, you need to throw money at them.
The one situation where DfT can effectively ban something is on safety grounds, but even then they can't completely ban particular stock, only mandate minimum safety standards. Since there's not really any major concerns with the safety of Sprinters and Networkers, they have very little justification on trying to force retirement on safety grounds. Pacers, on the other hand, they can probably make a case more easily about safety concerns, but the ROSCOs could respond by strengthening the existing vehicles.