It should not be forgotten that Cologne is the fourth largest German city and a considerable market in itself (add to that the region south and east of it for which CGN is the logical choice over DUS). If DUS can support 7 daily flights to BHX by BE and LH, it does not seem outlandish to assume that BHX can support 80 seats daily, given the fact that HLX and Germanwings did not fly empty planes, although they could not fill a 150 seater.
Plus, the BE flight opens up daily, reasonably priced connections from CGN to GLA, EDI and BHD (and IOM) which are not served from CGN at all / on a year round-basis.