Yes it is. It is not about the cost of comparing 2 products, it is about market saturation. When 2010 came, drug A would be off-patent and evidence for using A would be piling up. As a doctor that cares about his patient, one would very reasonably choose drug A over the still-on-patent drug D which is only slightly more effective but since it's new the evidence is weaker (as in -- will this work for the relevant genetic makeup, disease state profile, microbial resistance pattern for antibiotics, is this better efficacy real and not just some shady number crunching by big pharma etc.), nobody exactly knows the long-term side effects, and it's much more expensive. Do you think that the pharma would spend millions to face this bleak market?

(EDIT: this assumes that drug A works "satisfactorily". Drug D still have a chance if it has markedly differentiating features, say if drug A is injected while drug D is taken orally.)

(as a side note, cancer drugs are hot because of precisely opposite of these -- most are fairly new, those that are old are not exactly pleasant to take, there is no "silver bullet" found yet so there are lots of room for improvement.)

Another take on the market saturation is that when the current therapy is simply already good enough, ie. the "silver bullet" has been discovered. That's the reason why there has been no new drugs developed for headache in recent few years (or pain management drugs in general). It's a totally different story when, say, aspirin was first introduced: there is a lot more room to improve on / differentiate from its side effect profile, pharmacological properties, etc.