IBA eye-gouges on routes out of LHR once the competition leaves, LHR-GLA dropped frequency and prices bounced upwards once bmi gave it up, the consumer loses!
Options like CDG, AMS, FRA, MUC at least have alliance competition, MAD and DUB are IAG stitch ups, in previous years they would have been cartels.
Look at fares on Aer Lingus ORK and SNN by way of seeing how much they can charge on slots they got for free that commercially now no one would bid to compete on. Runway 3 could make such a difference to the consumer.
The competition comes in the form of Ryanair. ORK served from LGW, LTN and STN ~5 daily for less than £40 and SNN served from LGW and STN ~3 daily for less than £25, both on Monday.
Neither the UK's CMA and the European Commission's competition dept. have an issue with the IAG "stitch-up". Both regulators realise the market isn't LHR - SNN or LHR - ORK, but rather ex-LON as a whole.
You can argue the toss on whether FR are a worthy IAG competitor, but the regulator thinks so.
I also don't think a 3rd runway at LHR is going to change 'fortress LHR' into the oligopoly that you expect. Airlines feed their own hubs, there's no incentive for an alternative alliance airline to get in on the action between, say LHR and MAD. IB and BA cover the network traffic, EZY, FR, D8, UX cover the majority of O&D.
There isn't going to be a mad dash of new entrants ex-LHR, unless they feel the fares charged are worth committing scarce aircraft to that market (though I believe EZY will push to be the feed airline for non-IAG long-haul airlines out of LHR. I have my doubts as to whether it will be a successful strategy, however).