Thanks for all the constructive input. I can get over the history part that says " . . . because we've always done it this way."
DeadDMwalking said: "There's a psychology to saving throws.
If you roll an attack roll and kill me, it feels like it is completely unfair and I couldn't do anything about it.
If I roll a defense die and I fail, it feels like it's my fault because I could have rolled well and avoided it."
This is part of what I wrestle with. Part of me wants to be objective and stick with just mechanics, but part of me says the psyche piece is very real.
I'm looking for fast, efficient, and at least somewhat realistic for combat and danger (I know I'll never get completely there). I'm not doing a player rolls everything system. What I'm trying to create is d20, significantly more lethal than 5e, but not OD&D lethal, and hopefully a more streamlined rules lightish system.
Ideally I'd like none, one, or two save categories. The obvious choices are:
-None: Save is baked into the to hit roll.
-One category: Luck attribute
-Two categories: Will & Fortitude type groups
The other thing I wrestle with is that if I'm going to have saves, what qualifies to get a save? As others have said, D&D doesn't give a save when you get shot by an arrow, but does if a dragon breathes all over you. I feel like I'm missing the way to delineate between what makes some dangers qualify for save and others not . . . remember this is a roll to hit, even for spells, system.