Diminishing returns and % system means any and all bonuses you can add are useful - but get less useful the more you have - rather than now with the long mangled d20 system where many bonuses are either completely a waste of time, or way too useful once they put you over a magic threshold.
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Going to a d100 system is one thing, but the proposed change to 'to hit' is something else.
Diminishing returns is not a % system. In a % system +1 (or +1%) will always be half of +2 (or +2%) and one fifth of +5. In this system the actual percent value of "+1" varies, it scales depending on the AC and level of character.
Players will not be able to look at an item and intuitively know how much better a +5 weapon is than a +1 weapon because depending on at what level they use it and against what AC they use it on. Does the +5 weapon give a +25% chance to hit? Does it give a +20% chance to hit? +18.5%?
With the new formulas all you will know as a player is that +5>+1 therefore it is some unknown order of magnitude better. At that point you might as well just color code the items and toss out +'s altogether since the relative values won't be consistent.
And as long as you have power creep without hard caps on upper limits, they will need to keep 'normalizing' the game, and going to a d100 system is just a temporary fix.