There is literally nothing stopping casuals from switching specs whenever its needed. Casuals need to min-max the least, they don't have to be optimal. Its only in your heads. Even though the power disparity between covenants for most specs is clearly vivid, its only matters when doing the hardest content, or when you are trying to clear mythic while wearing hc gear. As for the rest, especially when people need to switch temporary - just do it. Soulbinds will still be useful, covenant ability can be less useful but not game-breaking. After all, it was never expected from people to play their offspec at the same level as their main spec. And legendaries - you don't have to craft 235 versions of them, anything below will still get the job done, you're only be missing some stats in ONE item.
Its actually the hardcore that got the short end of the stick when it comes to spec switching. its them who need to be optimal and level several characters of the same class for different covenants if it comes to it, or refarm renown when covenants get buffed/nerfed.
So you're looking at it from a completely different angle. Less optimal (not the best) =/= complete unplayable trash.