Most investment into UK airports has been into existing infrastructure not to pay off historical debt re the DART.
Will be a massive ask to expect a new investor in 2031 to take on a pre pack expansion plan from 2019 with the huge associated costs.
Certainly the (short) franchise agreements has been/is one of the millstones around LTN`s neck in trying for expansion.
Appears again to an outsider that Luton Council is/was too greedy in their income expectations & regard the airport as a cash cow perhaps.
For the massive investment/payoffs described it would need to be the 99 year lease,something Luton Council would with historical perspective perhaps find unable to grant.