You answered yourself there. The way current balance works and since you played cata you know this, it forces people to play specs that they dont enjoy or want to because the balance is atrocious. For pure dps classes, this isnt as big of an issue.
For hybrids however, that is another matter. Since you played cata, you know ELE wasnt viable, with the exception to every rule applicable, in really anything. Resto and ENH were so good and ELE so bad you had to play one of those two specs. It still works this way on retail (since last I played) with ENH and ELE switching roles. This means that those that want to play ENH for melee DPS cant currently, just like caster dps shamans in cata.
As you have stated in your post indirectly, Blizzard is not good at class balance. Now throw in a 4th spec for them to balance on one of the lowest played classes in the game with historically bad balancing⌠No thanks.
Never, ever, should they implement a 4th tanking spec for shaman. Absolutely not. I have played shaman since BC, so I would consider myself an old time player as well.