the 7900/9900 range is the better printer.
my belief is that Epson put the 7890 out there so they can compete with Canon's 6300/8300 on price.
the older head design and fewer nozzles means they have lower production price.
Canon with its economic bubblejet head design is annihilating Epson on price/value/lack of clogs, so they are repackaging older technology at a price point they can at least break even on.
Not sure what you mean by "older head design". If you are referring to piezo heads vs bubblejet, both are "old". If you mean older in that they've put the 7880 heads into the new x900 chassis, wrong. All the technology is the same as the 7900 including new heads, better dither. The heads are cheaper to make only because they have less channels of ink. Only difference between the two is the inkset, the expanded gamut of the 11 inks for photographic applications in the x900 printers may rarely offer some improved gamut, but really is designed to help the printer hit pantone colors that are not real world colors. The majority of users do not need those colors, and Epson has consolidated their production line offering those who don't need them a nice upgrade.
As far as the illusion of Canon annihilating epson, interesting idea, but unclear on what you base that. Canon remains a niche player in the large inkjet market, maybe finding a small window in the higher end photo market but still far behind and really not closing on Epson in the large format inkjet market. As a dealer for both (printers are not a product we actively promote, but we still sell a fair number each year), I have yet to have anyone ask about the canon printer. I can't even justify putting a demo on the floor. If Canon wants to sell them they certainly could provide one to run along side the 11880, a 7900, and a 3880 running daily.
Not knocking the Canon ... I've used one extensively. Good printer, and good output, and finally with the new ones it appears they've finally gotten close to Epson with GD and bronzing.