While I concur in the longer term, fleet renewal, especially for the larger “flag carriers” is much less simple.
You can’t simply retire BA’s 100-strong fleet of 747s and 777s instantly for a whole range of reasons.
In the short term it is much less expensive to engage in methods such as reducing luggage allowances, abandoning heavy in flight magazines, reducing the weight of paint on the exterior or introducing more direct routings, reducing stacking and developing direct approaches, as well as increasing runway capacity.
If APD was invested to deliver such benefits, I wouldn’t be so against it.
As it is, it’s an invidious tax which actually reduces airports’ and airlines’ profit margins at a time when they can ill afford it, and therefore stifles investment in greener technologies and infrastructure.