I've been told a few times by MFDB sales reps and resellers
that the highest megapixel products are the best sellers,
This is true, but it is due more to price on those particular products, (P30, Aptus 65, and now H3DII-31). What was exposed from the last few batches of digital backs is that the slightly higher premium for larger sensors (49 x 37, 22MP, Upper teen $ - low $20K) vs higher resolution (44 x 33, 28/31 MP, low $20K - mid $20K) was not acted upon. Instead, users chose the lower cost, but higher resolution (and more sensitive) sensor.
But also, until Hasselblad dropped the price on their H3DII-31, most of my H3DII sales were H3DII-39. This is because the price delta between H3DII-31 and H3DII-39 was not dramatic enough ($26,995 vs $33,995). This also explains to some degree why Hasselblad made such a significant price adjustment. Now I will expect - and have already seen - H3DII-31's selling in greater numbers than H3DII-39's.
It is human nature to want the latest and greatest. And despite all the "Oh, 22MP is more than enough" protestations when rumors of 30+ megapixel sensors were rampant, the vast majority of purchasers have shunned 22MP, even though it is relatively low in cost.
That said, 60+ megapixels and higher are on the way - at some point. I do feel that without some form of variable resolution selection at the capture stage, digital backs at this megapixel count may be the first crop of high resolution products that do not produce a volume sales advantage over their lower resolution counterparts.