My take is that the Ultra is an example of another iterative step in Apple’s quest to put a dent in Garmin’s, Suunto’s, Polar’s and (insert brand here)’s entry-level segments.
For running, the really early models did nothing more than measure the time elapsed, km/mile time, current heart rate and total distance (and some other metrics I cannot recall now). With the exception of HR, these are things most phones can do with the right app. Then Apple added (not in chronological order) pace alert, heart rate zones, custom workouts, and more.
At this point, the Ultra and/or WatchOS9 are becoming a very viable alternative to the likes of the Forerunner 45 and its competitors, and Watch owners who recently completed C25K or a milestone run and want to up their training will be even more persuaded to keep using an Apple Watch instead of turning to Garmin.
So my opinion is that besides people who don’t care much for the athletic features but want it regardless (more power to them - not my money on the line), other prospective Ultra buyers will be those who want some Garmin features from their entry-level models, and maybe a couple from their midrange models.