I think the very long stories are still being written in other fandoms -- look at the length of stories offered on Sweet Charity, for example, as well as the what, 40+ chapter multi-multi-fandom thing one author finished recently, immediately followed by a sequel.
They're not common in Highlander now, but that may be another indication of the fandom's fading away. MacG's WIP "Origins" probably qualifies as one, in scope as well as length.
Honestly, the Highlander "epics" usually cited are so badly written that I haven't been able to read them. The exceptions are stories by Becca Abbott and some by Maygra -- weird, weird characterization and buggy, but entertaining and not brain-scarring. I haven't kept up with Desert Prince, but what I read was well done. Sonia's serial set in Africa was fantastic and still holds up. Its publication as a serial kept me reading. Long (don't know if they qualify as epic) stories by Chris are also very nice.
I prefer reading linked stories to long, long fic. It's not a matter of attention span or time. I read long novels and series, even re-read them. For some reason, long fanfic fails to hold my interest. In most cases, it's because they're too loose or meandering. I do love the serial format, though, and I'd probably keep reading an unending tale told that way, in the right writer's hands.