I’d be ok with that, except there are some thematics that don’t work as just a visual change, like Rider of the Apocalypse.
In some cases, making them cosmetic only WOULD be liberating, and allow blizzard to make some specs cooler, like the suggestions in the edit to OP I just added, and let players choose to use or not use them just based on their theme.
In others, like rider, though, making them purely cosmetic limits their coolness. How could you make “permanently mounted while in combat” a cosmetic only feature without it feeling sluggish and tedious?
If they can somehow reconcile that, then yeah, I’d say go for it with the cosmetic class skins approach