Relatively speaking, 2 million for an EPD game is a failure. The reason why we’re caught up with 10 million sellers is because that’s what successful EPD games sell. You can’t divorce sales numbers from their expectations - and their expectations are based on the dev team, studio, and publisher. This is a studio that does 10 million+ games done by a team that does 20 million+ games.
I am not sure what criteria you are using as “low cost” when the cost was the time of some of Nintendo’s most commercially valuable dev resources.
Who expected ARMS to sell crazy numbers? It looked like a pretty niche idea from the beginning. And like another user mentioned; the pedigree or size of a studio does in no way indicate expectations of sales, especially if the budget is small (ARMS was not an expensive production and thus not a huge investment). With titles sporting more bloated budgets, higher sales expectations should follow. It should also be noted that ARMS reviewed well below EPD's greatest games.
If massive publishers like Ubisoft, EA, or Activision/Blizzard help develop and release simpler projects with low budgets, I won't automatically assume sales close to their best-selling franchises.
Wasn't Captain Toad: Treasure Tracker also made by EPD? It sold rather poorly on the Switch and had abysmal figures on the 3DS, the Wii U sales likely constrained due to the tiny installed base. Heck, look at something like 1-2 Switch, from EPD as well, selling 2-3 million, or Tank Troopers on 3DS. The point I'm making is; is ARMS a failure given its budget and the realistic expectations one should have from a new IP with a niche concept even if a studio with pedigree made it? My answer would easily be no, 2 million + is great for what it is and likely very profitable.