Oh man, I'm always a sucker for a well designed grind. I even played Maplestory because the grind was soothing and fun. Yes, it was terrible and eternal, but it was very relaxing and made a great environment for just hanging out and chatting with friends. I've called it the most complicated chat client ever before.
And my wife and I put several hundred hours into Dragon's Crown, a game in which you fight increasingly stronger versions of the same enemies in a circular or randomized order. The number of assets may be somewhat small and limited, but they are [1] beautiful works of art (the artwork in that game is a reward in itself <3) and [2] used in clever ways. Bosses gain new attacks or get faster and more aggressive. Enemies come in larger numbers and sizes. No matter how long you've been killing assassins, you're in trouble when you see a giant red glowing assassin and the lights go out. This escalation can go on, effectively, for as long as you play the game. The Temple of Mirages (with the most powerful and game changing gear) is procedurally generated, so it is only limited by how many digits the devs used to store the floor. There is an assumed limit of a bit over 63 thousand, but I haven't heard of anyone (even Japanese players who got the game first) being anywhere near that.
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"Listen deeper to the music before you put it in a box" - Tyler the Creator - Sandwitches