But you've missed the two big problems - airfield capacity and planning.
At the moment, stands are BHX's issue - there arent enough places to turn around enough aircraft quickly enough to utilise the runway to its full effectiveness. For the runway to operate at its theoretical max number of movements, they need far more stands. That's what the expansion to the south (something akin to the drawing above, minus the terminal (BHX's prior plan had a satellite pier where the northern remotes are, which fulfils the same basic purpose)) is essentially about.
But, once you've built that, your next blocker becomes airfield capacity. You can titivate a bit with rapid exit taxiways and the like, but your biggest problem is that the only way to reach the northern end of the runway is a taxi along a single taxiway with no airfield facilities adjacent, and no prospect of there ever being another taxiway or any facility expansion because of the noise mound and, more importantly, the houses beyond. That will always constrain movements, to around 30mppa. It doesnt matter how big the terminal is at that point. The ideas mooted above of works on the western side might help a little - particularly if there is a taxiway provided on the western side of the runway - but its not going to be a huge increase, and will nevertheless cost a small fortune to deliver (£2-3bn, i guess). And dont forget that the Elmdon side brings in all the cargo revenue which is booming currently.
And all of that needs to get planning permission . I see absolutely no prospect of planning being granted for wholesale change at the airport, particularly given the impact on the entire eastern side of the city. It may well be the case that the minor amendment to build more stands, because of its link to increasing movements overall (and hence increasing the potential mppa the airport can operate to), will struggle to get planning permission also (more pax = more flights = more noise). The government are currently looking favourable to that, though...!
All of this is why, imo, the long term solution is to move the airport, building it in a place where you can maximise efficiency. But the costs of that will be astronomical, and i dont really think there is demand for another 60mppa airport alongside Manchester/Gatwick/Heathrow. BHX would only grow to those sort of numbers if Heathrow (probably) was closed, which is vanishingly unlikely to happen. On that basis, it wont happen (barring a major change to government policy, which also looks vanishingly unlikely).
I dont think you can honestly put this down to a lack of imagination or vision from the airport. The time to have done something different was 30+ years ago, by lobbying government to move airport capacity away from London. That would've gone hand in hand with policies to promote growth outside of London rather than the policy that essentially sucked the life out of Birmingham.
N.B. I have just thought that one blue sky idea that might work on the current site would be to effectively shunt the runway half a mile or so further south, bridging the A45 and possibly the new link road (or getting up to the edge of). That, combined with the idea above of expanding the south side of the terminal might allow you to operate a far more efficient airport overall (no long taxi to/from runway end), going some way beyond the 30mppa current runway theoretical limit. Im not sure you'd ever be able to get to Gatwick type numbers, though.
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