Shovel Knight selling over 2M (Last time we heard) isn't big enough for this market ? (That's not counting the stand-alone campaign DLC) Sure, it ain't at the level of Stardew Valley, Hollow Knight or Undertale but it is still viewed as one of the most prolific stories in the indie market.
Considering all the hype, the 10+ platforms that it was released on and the 5 years of sales, 2.5m (which is the most recent number) isn't that impressive. Definitely good and better than the average indie game, but not among the top sellers. I could name a dozen that sold more just out the top of my head and there are probably a lot more, especially when looking through the Steam library where I'm not that familiar with.