The only reason thy replaced Carbuncle and Topaz with Garuda and Titan was because their elements overlapped not because of their role otherwise we would've never gotten Ifit as a Summon III.
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Doomtrain.
I don't get this line of reasoning, Carbuncle doesn't have an element traditionally. Carbuncle in ffxiv has the exact same set abilities as the summon counterparts just weaker. That is likely the reason why the elements overlap as you say, incidentally since they have the exact same abilities their roles overlap which is why Titan and Garuda supersede Carbuncle.
Anyway it's been confirmed that summon 4 and 5 are leviathan and Ramuh, surprising no-one. I hope Levi is a buffer pet and Ramuh a debuffer, only time will tell.
You do realise that is what I just said, right? Your first post said something about their elements overlapping which I doubt has anything to do with why they get superseded. Also Ifirt and garuda are two different types of attacker pets, caster/attack, that could (if ifirt was designed well) fulfill two different roles. We'll see what happens when the pets get rebalanced soon, I'm hoping to use ifirt and titan more.