IIRC, the concentration of slots at LHR held by BA/AA/IB is actually less than what Skyream has at CDG and AMS, and less than what Star has at FRA.
BA/AA do have a large piece of the market, without doubt, but it's not their fault that UA squandered whatever they had after purchasing Pan Am's authority... Plus, the brunt of the LHR market seems to be connections, and it's a lot easier for the other alliance partners to move traffic via hubs with less congestion and lower operating costs.
Also don't forget that with BMI, LH, SK, UA, etc.you have a pretty good presence from Star Alliance there as well.
The open market concept has worked out quite well for DL, who was able to get slots from AF, and for NW, who was able to get slots from KL. US and CO also managed to get slots, so pray tell, who is locked out of LHR?...
Mach, you're absolutely correct that BA could fly from MAD and IB could fly from LHR. But stop thinking of Europe as countries but states (which effectively they are now that EU has negotiated the bilateral), it's not that different than AA flying nonstop from California, Illinois, New York or Puerto Rico.
Plus, the notion anyone is going to be able to get away with using lower cost pilots seems to be a red herring. I won't say that there aren't going to be other attempts, but so far, it's been a failure for the few who have tried it.