the 3800 was announced in Sept of 2006, the 3880 was announced in Aug of 2009 (3 years), the p800 was announced in April of 2015 (6 years). I don’t think Epson is on a “cycle”. Each new printer was driven by improvement in ink technology, the basic design of the printer hasn’t varied much.
There was a lot of interest in better blacks and better longevity which led to the introduction of the p series of printers.There has been a steady improvement over the past generations regarding missing nozzles (clogs or more often air).
I’m not sure what would be next, maybe they can milk a little more gamut out of the inkset (which probably wouldn’t even show up in prints), maybe they can improve the head technology slightly (although the 3800/3880/p800 heads have been the most clog free of any printer out there). I suppose they could offer a dual MK/PK black system which doesn’t require switching, although if you don’t print with both inks frequently the non used channel can become very problematic (an issue with the original 4800).
Examine your needs now, but it seems to me you could be waiting a very long time for something that would yield very little or perhaps no benefit when it comes to print quality.