It also paid their staff market rate, a hell of a lot less than BA at the time.
Had it operated out of LHR (and maybe LCY and LGW) like the rest of BA it would probably have been as sucessful as the AF, LH, IB equivelants.
They're not successful.
Iberia Express exists only to terrify Iberia employees into taking major salary cuts to save Iberia mainline. Same livery, same owner, little difference to consumer, living proof IAG could close Iberia short haul over time and no one would really notice.
Germanwings has Lufthansa costed overheads and a Lufthansa unionised workforce, Eurowings is supposed to make this go away.
Air France's HOP is the latest incarnation of a bonfire of Euros in that space.
What BA did eventually do was shred it's service levels so far back and took and axe to it's cost base so much so that British Airways at Gatwick is now hat Go was at Stansted a decade and a half ago but still a UK superbrand and making BA money without cannibalising BA itself.