The sheer size and market dominance on key strategic markets.
btw you should change your username.
But LH/IAG will not be a monopoly on those "key strategic markets" since there will be competition from other airlines (most of all LCCs)
If there are any competition issues, then all LH/IAG needs to do is offer some slots here and there*
Also LH/IAG will not be the only big airline in Europe, there will still be Air-Frnce-KLM, Ryanair and Easyjet
I am not advocating a merger of LH and IAG, I am just suggesting it would be a good idea and not pose too many competition issues
P.S. Would you mind if you elaborate on those "key strategic markets" please?
*They did this for the LH takeovers of LX, OS and SN and the BA takeover of BMI