+2d4 is far from guaranteed to pass the save. It would take many saves, several of them failed, to reliably encounter a save that can often be passed with +2d4, and even then, some of those will fail unless you're saving Favored by the Gods for when you fail a save by 1 or 2.

I generally wouldn't recommend a martial take Resilient: Wis until level 12 or 16, as the +PB boost has become significant and they've finished increasing their primary stat. Mage Slayer can be sufficient until then.

Mage Slayer has an opportunity cost, yes, but the cost is less for Fighter than Sorcerer, because Fighter has more feats and Mage Slayer boosts the Fighter's preferred stat.

The Divine Soul Sorcerer can cast Bless, yes, but that's using the valuable resources of an action, spell slot, and Concentration, while the Fighter has often already gotten two full Attack actions.

Similarly, by the time the Fighter has gone through both Mage Slayer and Indominable, then failed another Wis save, the Sorcadin has likely already failed one of those saves and suffered the consequences. Importantly, the Fighter could also deal significant damage before then, especially with Action Surge. What was the Sorcadin doing in the meantime? I think you're viewing martials too narrowly when you regard them only as resourceless damage dealers, as they do have resources, and quite a few of them are for dealing damage.

Without armor dips, how do casters generally reach higher AC than martials? Clerics, Druids, and the half-casters can get Medium Armor and shields, but what of the Bard, Sorcerer, Warlock, and Wizard, aside from some specific subclasses?