While in a gaming funk this year, I tried a variety of idle games. They can be mechanically interesting, and they scratch that MMO Achiever itch and help you overcome it by taking it to its logical extreme.
“RPG” has come to mean “character advancement” and that familiar treadmill of leveling up by playing a poor combat mini-game to do the same thing with higher numbers and a more garish color scheme. MMOs have done a great job of pushing this to narrative irrelevancy through static theme park worlds that you cannot change because all the other paying customers need to be able to ride the same ride, and you can also pay to ride that same ride repeatedly. Fight goblins, then fight blue goblins, then fight orcs, then fight blue orcs, then fight gob-orcs, then raid gob-orcs until the expansion gives you a gear reset so you can start over. Along the way, optimize any remaining fun out of the equation in your quest for the most efficient path from number to number.
ProgressQuest is the trope-maker, a perfectly fire-and-forget single-player MMORPG. Create your character, and the game takes care of the rest as it slays monsters, loots their bodies, completes quests, etc. True idle games do their job too well: you get the point very quickly, and because there is nothing to do, you wander off. You might leave one running for a while just to watch it go, but it is too shallow to impact you meaningfully. Later idle games added gameplay, mostly something vaguely like economic gameplay in the sense that you accummulate money, use it to buy upgrades, and then earn money more quickly (and repeat). That can also be engrossing while providing the illusion of accomplishment and a perfect Skinner box of “push the button to get imaginary money/cookies.”
At some point, it sours. The part of your brain that knows that you are doing something compelling but not fun wins. You perhaps have a sense of letdown or betrayal. And then you cannot help but recognize how much “click the button to kill the goblin to level up the button to kill the blue goblin…” looks like “click 1 to shoot a fireball to kill the goblin to level up to cast Fireball 2 to kill the blue goblin to finish the quest…”