Except that life stealing aspect has always been a selfish form of recovery--something that heals the caster and no one else. With that in mind, it would make more sense for Necromancer to be a casting tank than a healer.
I understand what you're going on about with how not every job is a perfect replica of its past Final Fantasy representations, but I feel like you're taking those examples out of context. Summoner and Scholar are largely the way they are because of the decision to join them at the hip back when ARR was being developed. Their identities were both compromised to create the Arcanist that would link them both. There was actually plans to include a dedicated support role that would focus on buffs and debuffs, but was ultimately scrapped out of fear that not enough players would play Bard and that would elongate queue times even more. Moreover, Bard was adhered to Archer in 1.X because the role system there was far less strict, but moving that over to 2.X ultimately meant a lot of Bard's traditional identity had to be sacrificed. Dancer is the most support heavy job in the game next to AST. I'll agree that it's always been more debuff focused than buff focused, but seeing as how this game is designed, creating a debuff heavy job that doesn't break things is very challenging, so I understand why the compromise had to be made there. They were even given Cure Waltz to call back to their relatively frequent ability to heal.
Gunbreaker is not a traditional job and stems from 2 Final Fantasy titles that had either no job system, or an incredibly lax one (in which Lightning actually made for a very good dodge tank once Sentinel gets unlocked for her--arguably better than Snow at the role). In fact, you can see that despite this, the design team tried to hold true to specifically Squall's rendition of the gunblade with many attacks being directly named after his limit breaks. I have no idea why you decided to bring up AST though since that's never actually been a fully fledged job in any Final Fantasy until 14. In Tactics, Astrologian is not a real class, but a pseudo class that has only 1 unique ability, Celestial Stasis, which influences the name of AST's LB3 in 14. Saying that it has nothing to do with it's tactics iteration makes no sense whatsoever.
Lastly, we can look at how in other examples, Square is very adamant about holding true to a particular job's fantasy. They refuse to give White Mage any form of support since that's not the traditional fantasy of a classic White Mage. Red Mage was given a melee combo and Vercure/Verraise specifically to ensure that it held true to its origins. Hell, just look at Blue Mage. They were so unwilling to diverge Blue Mage from it's origins that they decided to make it a mini game rather than an actual job solely so that it can learn spells from overworld enemies, most of which are gimmicky nonsense like Doom and Level 5 Petrify.
At the very least, each job that has diverged from its original source material is at least playing within the same realm of its initial design--The Summoner job by itself is a damage dealing spellcaster with heavy burst damage; Scholar is typically supportive, though not a potent healer; Bard has at least typically had some form of burst damage even in FF3 (where it literally has a song that deals % damage, including to bosses), and if we take Bard for what it more accurately is: Ranger, then it makes even more sense having actions that hail back to the Ranger such as Barrage; Dancer has always done a little of everything.
Saying that we should take a job that is defined by its association with bad status conditions,death magic stronger than black magic, and selfish HP draining and turn that into a benevolent healer makes absolutely no sense.
Fine, you want a healing Necromancer? Go for it, but then I want an Assassin that's a tank because a job associated with stealth and fast, powerful damage should be a low DPS dodge tank, and then we should add Beastmaster, but we can make it a magic DPS and we'll say that they're just magically conjuring animals to attack.