Solo is a really interesting topic in an MMO. I've been back and forth on the whole subject. Having experienced it in a variety of other games, and the different forms it can take, I've arrived at the conclusion that it really depends on the purpose of the solo content. For a game like FFXIV where the story is locked behind dungeons/raids and that's one of the biggest selling points of the game, solo almost becomes mandatory. WoW has very little of that, and that's not the main appeal of the game, so it's less important that players are able to access a dungeon solo.
That said, I think there are some dungeons and raids in WoW that are incredibly iconic and should get the solo experience, even if an abridged version for solo. I think giving players the option to dip their toes in the actual multi-player content of WoW at a risk free level to start would be a great way to encourage people to try the MMO side of things.