You might have a high save (or even an 'auto-success' buff) but even that can be overwhelmed given enough debuffs and conditional penalties.
But that's how the game
already works without any need for complicated or ill-thought out rules.
Spellcasters can learn a wide range of Spells, even the Sorcerer can afford to learn alternatives like a Sleep & Color Spray or Web & Glitterdust. But more importantly is that they
pay a debuff of versatility to overwhelm targets that may be immune to one of their choices. They can even use conditional modifiers, like Energy Substitution or Searing Spell, to bypass stuff like immunity to Fire or simply rely on buff/debuff effects to compensate for immunities, like using Golem Strike to Sneak Attack Undead.
Also you're forgetting the generational gap. The creator, and his crew, ran some very deadly games because that was what was fun to them. The lethality built into D&D, such as cats killing commoners or Wolves murdering 1st level Fighters, Massive Damage, SoDs, Spheres of Annihilation, are intended. Some of the most powerful effects in the game, like wish or Ice Assassin, are designed to make you question using them and them rationing out rather then spamming them as your solution to everything because you're not supposed to have easy answers.
This also has a lends to the social dynamic. Without character death you won't make a new one and so you lose out on several opportunities to pretend to live out new lives from new prospectives and mesh them against other characters being roleplayed on the tabletop. Certain games are even designed to capitalize on this. Like Hackmaster is known for it's very detailed, and randomized, character creation is specifically designed to break the player out of his predetermined mold and try something new.
But like colleges & campuses everywhere. It's no longer about testing new ideas and learning from them but the suppression of dissenting opinions. Newer generations cannot stand to lose or be wrong about anything so no character deaths ever and "DMs" often dismiss anything that disagrees with them like some kind of tyrant that decides what is acceptable or not. Like take any Alignment or Code of Conduct discussion, no one wants to learn how to run a character with certain behavior traits, they just want to argue about how useless the subject is until everyone stops talking about it. As a result no one is testing their concept of morality or hearing opinions on it, rather they all mutually agree they know already know what's best and the other guy is a hateful bigot that could never learn the right way of doing things.
And I feel like I've ran off on yet another SorO rant about how you kids need to grow the heck up. Where was my addled brain wanting to go again? Oh yeah, 5th Edition. Why limit your self to 3.5 if you're just going to complain that it
needs you to make it right. You should consider all the optional available and try to grasp the full picture of them. Maybe 5th already fixed SoDs like you want, maybe the 3.5 already handles SoDs like you want but you were looking at things from the wrong angle, maybe SorO has a point that D&D may not even be the tabletop game you want to play and hopefully someone in the community can point you to one you'd like. Because pragmatically, why invest a ton of effort fixing something when there is already something that works?