The issue with the airport operator's reluctance to spend out on the DART is because of the franchise structure. Why would they shell out £200m plus when their franchise, and their chances of recouping their outlay (never mind making a profit) expires in a few years' time. I am unaware of any other item of national infrastructure that is based on this flawed model.
If you accept the premise that the council taxpayers of Luton have no responsibility to provide an important part of national infrastructure, then you should accept that a new owner is preferable. That owner, whether public or (preferably) private, can plan for the long term in the knowledge that their investment can be recouped either through trading or resale.
It's also interesting to note that
AFAIK the "franchise" has never been contested, merely extended.
In addition to that, local authority ownership brings with it positions of authority for local government politicians. People I wouldn't trust to run a bath, let alone an airport.
It's time for a proper trade sale, to allow the council to recoup their outlay and repay their debts, and use the balance to establish a kind of "sovereign wealth fund" for the benefit of Luton council taxpayers.