As soon as LH announce a new route out of DUS/FRA/MUC then BHX has a new route, as soon as KL announce a new route out of AMS then BHX has a new route, SK out of CPH, SR out of ZRH, SN out of BRU, AF out of CDG etc.
Your point is fine for long haul, you can travel the world from BHX with one simple change at a major European hub, the range of full service carriers at BHX is strong compared to other UK airports (and I for one think that there are few other direct long haul destinations that could be served profitably from BHX even with a runway extension, save perhaps India and another Middle Eastern carrier). But your statement does not rack up for short haul European flying.....take Berlin (I know, I know its been done to death!). My sister lives out there, by flying with any of the carriers you mention involves a transit stop taking the overall journey time to a minimum of 4 hours or so and at a much higher fare than flying direct from BRS or EMA. Even with the road journey to these airports its still quicker than transiting in Europe. I do always check for fares from BHX as it is by far easier to fly from there for me but the cheapest for my next journey in Feb half term was with LH at nearly 200 quid. So yet again I am off with Easyjet from BRS, 98 quid all in incl credit card fee and hold baggage. Its the same for Krakow at Easter, LH ex BHX £220, EZY ex LPL £86. Two journeys, only one person, yes granted that does not constitute a reason to start flights to either of these places but I bet there are an awful lot more of me living close to BHX out there flying from other airports instead of BHX to get to where they want to go.
The irony is that the airports I have mentioned 10 years ago boasted very few of the European destinations that BHX did serve, yet now they do and BHX does not. Something somewhere has gone very badly wrong. Hve EMA, BRS and LPL got stronger inbound tourist destinations or have they not suffered as badly in the recession? I do not want BHX to become dominated by one or two low cost carriers but at the same time would like to see more European flights. Maybe solid and dependable BE with their new EMB175's could be the answer (I can't actually think of any other answers!)