My thinking here is that (G)BR introduces a "pathfinder" unit.
Taking Hull as the obvious example, run services for a couple of years to prove that the demand actually is there. Then, fold it into the main operation; which helps with stock utilisation (in the case of the 80x units), ticket acceptance, onward connections, the train plan, etc.
Lumo is slightly different. They're more like a reverse Seatfrog (or Tesco Mobile), in that they could sell spaces on existing services at a lower cost in order to fill them.
(Come to think of it, what happens to Seatfog in the Brave New GBR World...?)
The idea of a pathfinder unit for GBR is IMHO an oxymoron. The point about Hull Trains was that it took genuine risks.
I can't really think of any open access operator that has succesfully "built new markets" other than Hull Trains, which is also 20 years or more old a this point.
If there were lots of other gaps in the industry I would think that the open access operations would have appeared by now - so I'm not really sure I buy this argument.
Lumo might bring in a minor amount of new business by abstracting from domestic airlines on the Scotland route - but domestic air is not the major competitor to the rail industry, so I don't think it really matters much.
Competition within the rail industry might be a benefit to the small portion of the population that regularly uses long distance rail - but that isn't the same as saying it is in the public interest. And it is the public that pays for all this.
Personally I'm strongly in favour of vertical integration of the railway industry, and I can't see any way that that is compatible with the likes of open access operators running around.
The public interest is broader than you allow. And, while I support vertical integration, the railways have always had to deal with penetrating services (e.g. L&Y services to York). Rigid vertical integration may work for the railway, but it is far from in the public interest.
More broadly, the railway industry has very high barriers to entry. It requires a lot of time and cash to get to the point of being able to launch (note the failure of Go Op), and then requires a lot of expenditure on people and kit to run a service, which isn't guaranteed to succeed and may take time to earn back the initial investment. Until the tail end of the last government, policy was hostile to open access, so the encouragement was slight.
Ultimately, the point is not whether a specific market is new, but whether the overall market for rail grows. And there, we have to acknowledge that the rigid approach of DfT to franchises even before Covid left increasingly limited room for entrepreneurial initiative.