It's always felt counter-intuitive to me that the 'bigger rewards' such as skins, personalities and body markings come from lesser achievements, while harder achievements give a title at best or nothing. I definitely wish there was some kind of reward for the trifecta achievements in dungeons. BUT cosmetics are one of the most desirable rewards ingame and locking skins and the like behind the harder achievements just isn't feasible. I think titles are a fair compromise because while they may still be something that's desirable from a cosmetic point of view, they're not as big of a deal as skins, personalities and body markings. Titles also, IMO, fit well concept-wise as something that lets you show off an achievement.

So yeah. I definitely wish there was SOMETHING for dungeon trifectas, just like trial trifectas give you a title. While I do personally would love the feeling of earning something bigger from these things, I don't think it's fair to the rest of the playerbase and thus not realistic. I think us people who enjoy that content need to placate ourselves with the fact that we are doing it for the sake of the challenge. There's unfortunately too many people who wouldn't go near that stuff, and too many of those are (much like me) way too passionate about the gotta-catch-them-all dress-up-game side of ESO.
I'm fairly sure the reason the vSS mount isn't a bigger deal is only because a: there was a very similar crownstore version and b: it's not popular design wise.

So yeah, I really get it. And if there's a way to implement it in a way that's not unfair to other players, I am all for it. But I personally can't think of a way to make it work.