"Now put the loco bases on a map and add driving catchment areas and the justification of many non-loco served airports just isn't there. On that basis 90% of air travellers are caught by LHR, LGW, LTN, STN, MAN, SOU, BRS, BHX, NCL, EDI, INV and ABZ."
Should LBA be on that list? Or does it also need to diversify more?
I think the answer to your question is no there is simply not enough other aviation related activities for all these airports to diversify into.
I suspect in the next few years a few more will become non airports, though aviation may well continue. DSA is in danger as is MME, mainly due to an owner who bought them cheap and intends to profit from the land they sit on as it did with Sheffield City. Sad but there are too many runways other than in the London area, but that's another topic!!