If BA and LH were to merge OneWorld and Star would not accept a member who is also in another alliance - there would be too many conflicts of interest. Thus you end up with one of the alliances going from strong to puny in Europe. Everyone would realise this including large corporates and other airlines. The result would be the weaker alliance degenerating and collapsing as corporates moved business to other airlines and airlines switched alliances. The result would be just 2 global alliances and not 3. For that reason alone a BA-LH merger is not hoing to be permitted by regulators regardless of slots given up.