Like I explained before, diminishing returns encourages sameness unless a lot of other thing aren't allowed.
If going past 16 is inefficient enough and lacks a strong secondary reason to go past, no one might ever go 16.
A dwarf fighter starts with Str 16. He'll put all his abilty boost from levels to Wisdom since doing so to Str grants no bonus. If he gets a +4 str item, he might not even use it. He'll trade with the rogue who has the +4 dex item (who doesn't use the +dex item for the same reason). He won't ask the wizard for Bull's Strength, Cat's Grace, Bear's Endurance (because his's a dwarf), nor Owl's Wisdom as none of them will give him a bonus. At level 10, every fighter has ~16 in their Str, Dex, Con, and Wis ability scores regardless of race because it is inefficient not too..
Diminishing returning is a soft cap. Soft caps are hard caps for people with system mastery 90% of the time. Hard caps encourages sameness. seen in happen in video games, it'll happen in d&d. Even the New York Yankees don't want to be over the luxury cap anymore
.