Well I hope that they have £153m laying around as that part seems to only have its interest serviced, also as you say times have changed and the lender may not wish to refinance!
Also if the money for Teesside is written off there is still the asset and it was upto the mayor to put that money somewhere! Basically the mayors office (taxpayer) owns the airport. If it makes money they will get that money back to be reinvested in another project.
At Teesside as long as the business plan goes to plan
the taxpayers will have that money back to reinvest.
At Newcastle if they paryback the loans it goes to a commerical lender and not back to the area to be reinvested!