I mean, if Holy Paladin, Disc Priest and Resto Shaman is the prefered healer trio for a decade, then yes, we should balance that.
It's not like this is a complicated issue. The core problem is simple - these three specs bring in vastly superior utility, be it damage reductions, SLT bypassing various healing reductions and absorbs inflating raids hp to crazy degree. The other three only have their raw output - and this output isn't really much better, to the point where you "need" them.
Also, it's not like their usefulness drops beyond WF guilds. Spirit Shell was absurdly strong across most skill levels. SLT would still keep people alive on Generals with -100% healing debuff. Aura Mastery offers a good damage reduction at long range. You generally don't go "pff, we don't need that", but rather "we don't have these specs - it will make things harder but we have no choice."
'Course, this is yet another expansion where DRs remain the same, so I really doubt anything will happen. Hopefully Symbol of Hope rework is a sign of things to come, but it's probably not strong enough to influence things yet. We'll see how it goes in further patches, but I don't think this WFR will have any changes to core healing roster.
I wonder if things would work differently if Ion actually played a Mistweaver or Resto Druid, instead of Shaman.