This is all incorrect and has less to do with jobs capabilities and the dev team and more to do with the playerbase.
SCH and RDM are not made to replace WHM if it isnt there.
They are not healers. They are hybrids with RDM having more melee/enfeeble focus and SCH being more casting focused.
SCH has decently capable nukes and can increase their accuracy with abilities, less mp cost, and some fast cast. Yes I know someone earlier mentioned how they hate books used as a reason but that is your unique ability, learn to use it more effectively and you can make up for "short comings" RDM just has never been allowed to melee...ever it is the only job that can have room to really complain, especially with the decision to give it high enhancing skill and not giving it unique buffs or AoE pro/shell.
If you need a tank you do not pick THF unless the mob is easy and does mostly physical attacks, if it nukes a THF is dead.
It does not have the shadows or higher INT of NIN to block those and cannot enfeeble or use /DNC the same way to survive. WAR has the ability to use MANY weapon types and off tank, playerbase won't allow that.
DRK's have just stopped using their magic completely and have the advantage to SC and MB....something people just rarely do anymore, it can also enfeeble and absorb attributes but doesn't, again playerbase fault.
When I start seeing more SCH's use their books and helix spells to boost their skills then maybe I can understand the want for more, but few do. However I agree with the idea of RDM getting more unique and AoE buffs or a buff/heal and giving SCH a super heal helix cool thing instead of Cure V.