I agree with this, with diminishing returns you’ll never hit that 100% but lower difficulties wouldn’t do much to you as you’re just a bad-A in general. Higher difficulties will still pose a problem, instead of just getting to a stopping point of saying ‘Well 75% damage reduction against a monster that does 400K damage is still doing 100K to you and one-shotting you anyway.’
At some point you will reach that problem, but that problem is pushed farther away with diminishing returns on armor and resists. Assuming you wish to take your character that route of course, all about that risk vs reward.