I thought it was more to do with the DfT specifying it because PDL commuters keep going on about it? So simply forgetting about it, whilst in any other case would probably be alright, would in this case still leave a problem to be solved?
That’s not how it works. Uually DfT only specifies rolling stock if it has procured it itself (Class 800s, Class 700s) or it is subject to special agreement with the lease company to roll on to the next franchise.
What normally happens is tha DfT specify the capacity required in terms of the minimum number of seats / passengers on a certain route in a given time period. This can be very detailed, and could include seat configuration on certain routes. Franchise bidders then respond to that requirement with proposals for certain types of train / formation / service level. The winning bidder then enacts it.
For in franchise* change, a franchisee won’t change anything that affects its contracted commitments unless the DfT asks it to. The franchisee may prompt the DfT to ask, but the DfT must still ask. Pure speculation here, but the DfT could have asked how SWR could save some cash at minimal impact to passengers, given forecast demand. SWR would then have replied with this as a proposal, perhaps on its own or in a ‘menu’ with others, all priced. DfT then instructs what it wants, taking into account costs vs benefits (not just financially, either). Decisions above a certain ‘size’ have to gain approval of the Secretary of State before being instructed.
* franchising is dead, but you know what I mean.
Plus it'll be a promise swr has delivered upon, and they need some of those.
Strictly speaking, a contractual requirement, that may have been rescinded.