Now veering into the meaning of liff I guess.You pose questions no one on here will or can answer absolutely.
Generally with your quoted expectations...ahh if only it was that simple.You too could name at least two other airports with new shiny terminal buildings which are woefully underused but had great expectations..(that`s two book titles mentioned now BTW).
Because LCC`s have doubled in size does not extrapolate with & according to local wishes & aspirations either I`m afraid.
Lastly BOH being 20 mins flying time from London too is no justification to hope or expect....& anyways 20 mins for the aeroplanes but perhaps a little longer for the SLF.