Dunno about this one, I mean don't get me wrong I expect it to sell very well, like 7-9 milllion, but over 10? Then again I'm the genius who thought the Switch would sell less than the Gamecube so what do I know.
I'm not too sure either, hence why I labeled it as 50/50. Luigi's Mansion 2 was at 5.75 million be the end of December 2017. Furthermore, Luigi's Mansion 1 was at 3.33 million be the end of December 2014:
https://www.resetera.com/threads/nintendo-software-and-hardware-sales-data-from-1983-to-present.2725/ So, I'm guessing that it's getting close to 6 million at this point.
So, that's a jump of around 2.5 million from the 1st to the 2nd game. So another leap like that for the 3rd game would bring it to around 8 million. I think it has a reasonable chance for two reasons. 1) The trend with other Nintendo franchises getting quite the jump in sales from their previous system to the Switch. 2) It's a Mario-related game, figuratively and literally because it's Mario's (far more interesting) brother, and just about anything that has Mario's name or likeness attached to it on a successful system like the Switch is guaranteed to sell huge numbers. Time will tell though. It would certainly help a lot if the game is released and given an overwhelming positive reception and critical acclaim to help boost its legs after launch.
Pancho A. Ovies
Nintendo Switch in Japan (Famitsu): 2018 vs. 2019
PlayStation 4/Xbox One/Nintendo Switch: 2018 vs. 2019