Why not just give it to them till HS2 & be done with it? They seem to be doing a good job & passenger numbers are still climbing. Saves all that bidding grief & cost for let's face it what's a very short franchise. I
That's not a bad point. Even with no big changes/upgrades it is as you say ticking along reasonably well and seems popular with passengers. Not a part of the railway that really needs any faffing about with.
One of the main reasons for the refranchising is so that the next franchisee can lead on designing and then operating the HS2 service (it's not short it's supposed to run to 2031 with an optional extension to 2034!). It's why bidders are required to have a strong High Speed Rail experience on their team. I have to say as an approach I would suggest it has a lot of merit. We have a very complex piece of infrastructure to deliver alongside a significant change in the services on the busiest railway line in the UK and so I'd suggest it's far riskier to, at the same time, have a franchise change in the middle of that!
There's not much that makes sense about franchising and the way that the DfT go about it but this is one of the few things that does make sense.