Of the 3, Cranfield is the only viable option. Duxford lacks the infrastructure (runway length, taxiway widths/PCN, ATC services, controlled airspace and Instrument Approach aids/TAPs), Wyton is already earmarked for development. Cambridge has long been the Marshall family's "nest egg" - just a matter of when the business need and potential land value / housing need conspired. As I understand it, as Marshalls is still very much a family business, the interest in Aviation waxes and wanes depending on who is in the boardroom - and it changes between generations. Cranfield is benefitting from significant investment - they are lengthening the runway, have the first "digital" tower in the UK and are expanding the capacity for other users through the Air Park initiative. Alconbury is, it seems, too far gone and Mildenhall appears to have some form of stay of execution. IMHO, the inclusion of both Duxford and Wyton are part of a smokescreen to buy time for Marshalls to get their
PR strategy up and running before they move out of the Cambridge area.