I've always had mixed feelings about RPG's reducing the amount gained per level up just because the player levels up a lot. Just to summarize, diminishing returns are when the player levels up a stat over and over again, but the benefits of leveling it up decrease as well as becoming more costly with each consecutive increase in level. So, lveling up HP the first 10 times will provide more hit points than leveling it up for the 40th time.
Personally, I'm the sort of person who enjoys maxing out everything for the sake of completion, and because I like to feel OP. However, this hinders my progress because leveling it up all the way doesn't feel very awarding, just a chore in order to get it done.
These mechanics also mess up PVP/Multiplayer in a lot of games. For instance, in Bloodborne, players can't fight each other unless they're close to the other's level, so that a level 100 person can fight other 90-110 players but wouldn't be forced to fight a level 40 or 300 person. This may seem fair, but with diminishing returns, the player who levels up 80 times in health and 30 times in their damage stats will be weaker than the player who spends 80 levels amongst their damage stats and 30 times in their health stat. A level 150 person who has maxed out only 3 stats would likely be just as powerful as a level 90 person with a balance between multiple stats to have more versatility.
A lot of people seem to really think diminishing returns are smart ideas, but I can't see why. Perhaps someone has their own testimonies on this that I haven't considered?