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It's one of these bizarre things which says a lot about the British. Those who fly a lot, and who are familiar with various hubs, rate LHR highly. ( I remember T5 winning awards.) Those who rarely fly seem to only remember the bad publicity from several years ago and avoid it on the principle that things are always better elsewhere. Hence KLM have feeder flights from almost every UK airport, and Air Lingus from many.
The industry is rather more complex and there are many disruptive forces at play these days.
Reference the ME3 Turkish and the EU golden triangle alliance hubs (For clarity that is defined as FRA/AMS/CDG) all offering global one stop long haul access from the regions at frequencies way above the historical levels of the late twentieth century.
Whilst the flexible fares (so called LCC or ULCC what ever that means) carriers are providing levels of EU and Mediterranean flight opportunities never ever offered by the legacies equally from the regions.
Though these have almost killed off the traditional independent charter companies as the traditional package tour goes the way of the dodo.
Many many more people of differing economic classes fly today from/to the UK multiple times a year and simply have no need to transit Heathrow, after all most of these will be inter Europe both leisure or small business trips where unless you are chasing mileage or for some other commercial reason LBA- LHR- DUS for instance is simple barmy.
As for both KLM and Aer Lingus they have been operating into the UK regions for just about as long as commercial passenger flying has existed !
And both have very substantial O & D traffic levels in addition to the feed.
Indeed the KLM east coast routes from Norwich/Humberside/Tees-side/Newcastle and Aberdeen can be some of the most expensive routes for point to point as much is petrochemical related traffic.
As for Aer Lingus they have offered US transits through Dublin since the fifties yet most of the traffic remains point to point commuters for most of the day.