You seem to be associating a few different things as one, but to help split them out a bit:
1.) Bethesda Games Studios is still, at lost, ~200 people, and it would be surprising if they had more than half their staff on one project given they hacks three concurrent project. We would be looking at something more on the Quake Champions/Titanfall/Payday/Dota scale than the Destiny scale (which is 1100+ people).
2.) There's also a pretty extensive difference between traditional MMOs (a la WoW or SWTOR), Destiny-type-game (these play like Diablo or Borderlands, and Star Citizen/Elite Dangerous (which play like Tie Fighter or Freesoace). Like those three categories aren't really competing for the same audience, and even when they are, they aren't substitutable games unless someone has just stopped liking the old type of game, like how someone who play GTA might stop playing Gran Turismo since they just liked driving in games instead of sim racing. That's kind of a different discussion though.
3.) By the same argumentation structure you're using, presumably Bethesda would be concerned about launching a singleplayer sci-fi WRPG because Mass Effect and Deus Ex tanked to the point their series got put on hold, tangentially related products like Prey, The Surge, and Star Ocean tanked, and Fallout 4 had notably worse legs than Skyrim. There's also CD Projekt's game coming up for competition (assuming that has a notable singleplayer component). By this type of grouping, it's both a crowded space, and an underperforming one. Of course, one might feel this type of grouping is invalid for raising the sales alarm bells, but I'd then propose the initial concern doesn't make much sense either.
