Legend of Grimrock situation is not a unique one. A lot of sequels tp indie games are nowhere near as successful. A few more examples:
Toki Tori: 221,865 ± 26,939 (55.32%)
Toki Tori 2: 82,773 ± 16,459 (29.22%)
Costume Quest: 200,532 ± 25,612
Costume Quest 2: 17,067 ± 7,475
Defense Grid: 436,903 ± 37,786
Defense Grid 2: 52,053 ± 13,053
Audiosurf: 1,131,510 ± 60,716
Audiosurf 2: 91,306 ± 17,287
Puzzle Agent: 112,639 ± 19,199
Puzzle Agent 2: 42,666 ± 11,818
Anomaly: 415,570 ± 36,853
Anomaly 2: 80,213 ± 16,203
Zeno Clash: 239,785 ± 28,005
Zeno Clash 2: 103,252 ± 18,382
Cubemen: 116,052 ± 19,488
Cubemen 2: 52,053 ± 13,053
I know the comparisons are far from perfect, because it doesn't take into account the number of times each game was bundled or was on sale, and the first game is obviously on the market for a longer period. Still, for all of the franchises above, I don't think we will see another sequel.