Pete, I have been using my 7800 for around 12 years, near as I can figure. I am not a pro, but a serious enthusiast who sells a print from time to time. In that time, I have gone through all manners of "box" gear - Contax G > Nikon D70> Rollei MF > Fuji LF > Leica M9 > Leica S (006). With the film gear I have used a Nikon 9000 and Eversmart Pro II flatbed scanner. My point is that as I have moved up the file food chain I have watched very carefully to see if I have ever exceeded the reproductive abilities of the 7800. And I never have. As the input gets better, so does the output.
Over the years, I have contemplated the same question you have as I read the promotional materials on newer generations of printers- am I missing something by printing with a 12 year old rig. Common sense suggests that I am if guided by the assumption that newer must be better. But I have never read anywhere that there are decisive image improvements with each new generation of Epson printers since the 7800. They mostly seem to be tales of woe.
Moreover, I am an episodic printer. I can literally go months without firing it up. And then I will make several prints. While the machine always tells me that I must do a power cleaning (remember Epson is an ink company first and foremost), I will first print a test pattern. Even after sitting inactive for several months, more often than not, I get a perfect pattern. I have never had a clog that would not clear with a couple of print patterns. I have never replaced a head nor have there been any other mechanical or electrical issues. I now have my workflow all calibrated. After futzing around with a manners of papers with mixed results I am now printing very happily on yeoman Epson Pro Semi-gloss which looks great under glass - equal to the Ilford and Harmon papers I have lusted after. With the stock profile and calibrated NEC screen, I am getting nearly perfect screen matches the first time.
When I contemplate all of the gear I have replaced over the years I truly have never had a "need" to replace the 7800 even if I have a "want" fueled by the blind assumption that newer must be better. But the more I read about the newer generations of Epson printers the more I respect the dependability and capabilities of the 7800. In fact, if I were to do anything, it would be to buy another one as a dedicated B&W machine, just as Jon Cone suggests. If you do end up moving to a new platform, make sure that whatever improvement you might discern in image quality is actually perceptible under glass in hostile lighting environments. And that it is worth jumping from a solid, proven platform to the unknown. After all, no one looking at your images will give a crap what printer they come from. Only if they resonate or not. Let us know what decision you make.
Cheers,
David