My guess is that part of the reason for this (once you strip away the politics of it - the left wing "nationalisation" versus the right wing "nostalgia"), one of the pragmatic reasons for this is to try to get the costs down.
We've had various different franchise maps (as the balance changed from tiny TOCs like Valley Lines/ Island Line/ Gatwick Express to mega ones like TSGN, but also seeing new ones like Caledonian Sleeper), but there's been roughly two dozen franchises.
However, over time, the number of firms bidding for them has contracted and the number of firms winning them has similarly shrunk. Look at how exotic the original bidders seem now (Merseyside Transport! PRISM!), whereas things became so complicated and onerous that it became restricted to only companies with significant existing railway experience (generally state operators from other EU countries)
Then it became too much for even Stagecoach and National Express (which meant we lost the Virgin brand too)
That's not sustainable - I'm not suggesting improper behaviour but Abellio and First have so much market share that they could significantly skew the market, making franchises much poorer value for taxpayers.
So my guess (and I stress, guess) is that the Government will want to try to encourage a range of bidders to create a competitive market - small enough to get a range of companies to bid for them - keep it competitive - stop anyone dominating the market.
A quick check of Wiki states that London buses were initially split into eleven companies, and buses in London are currently operated by:
Abellio London,
Arriva London,
CT Plus,
Go-Ahead London,
London Sovereign RATP,
London United,
Metroline,
Quality Line,
Stagecoach London,
Sullivan Buses,
Tower Transit and
Uno
Obviously some of those operators are predominantly focussed in one part of London (and therefore an east London based operator is unlikely to bid for a contract on a local route in the Heathrow area), but that seems to be enough operators to keep the market pretty competitive (and simple enough for a new provider to get involved if they want to - there aren't *huge* barriers - if you could lease a depot and some vehicles and rustle up some drivers then you can bid for a contract. That seems a reasonable blueprint for heavy rail.
Trying to leave politics aside, my guess would be that the Government would want to package things into smaller contracts, which would encourage some new companies to get involved (or even some "old" companies to come back into rail) - look at how many third party outsourcing companies there are out there who've not run passenger trains (but might be able to do what's asked of these new contracts - e.g. you're not doing infrastructure, you're not planning timetables, you're just managing an ongoing operation, which is what companies like G4S/ Mitie etc do - you don't need to know much about trains, these are more like the management contracts that the likes of G4S tender for)
I don't even think that we'd need to change the overall operator branding - e.g. there's no reason why we couldn't have Company X running Northern's Heaton operation, Company Y running Northern's Neville Hill operation and Company Z running Northern's Newton Heath operation, all with the same general Northern branding - just like you can have different companies operating all of the Subway restaurants in your city with practically no way that customers would know/notice whether they were run directly by Subway or run by the same franchisee or run by several different franchisees - they have certain common standards, they would be fined if they didn't maintain those standards - same with McDonalds franchises and others - you give them standards and they have to live up to those standards if they want to keep the contract.
