Just to name a few games in Japan for the Switch which are out or come out before Fall:

Zelda

Mario kart

Splatoon

Dragon Quest 1+2 and X

1-2 Switch

Snipperclips

Fire Emblem Heroes

Puyo Puyo Tetris

Bomberman R

Minecraft

Seiken Densetsu Collection

Fate/Extella

To be fair mostly people were not interested even for Splatoon before 1st Test Demo, Test Demo is what made interested in Splatoon explode, fact that people itself actually tried and saw itself how much great game is.

Yeah because people totally buy a system first and only afterwards look and see what there is to play on it and not the other way around, that's definitely how it works! Your numbers don't even make sense with that logic, as it shouldn't have strong legs with more games releases overtime to compete with for that supposed "because there's nothing else" spot.

And yet you implied that they do.

I feel like ARMS might have a soft opening but word of mouth could make it a nice hit in the long run(LEGS, yes). The testpunch could boost interest. I also wonder if the release of Splatoon 2 will help it as it seems like a perfect complementary purchase for people buying a Switch at that exact moment. If it's as good as it looks it could easily be an evergreen seller.

So, the prediction:

80k first week
320k by year end

I hope I'm undershooting but it's hard to gauge interest. One thing going against it is that it's not immediately obvious if it translates well to handheld mode, and in Japan that could make or break it.

I'll be cautious as I think this game will perform much better in the west:

FW:82k
LT: 350k

In the west I believe it will do 400k FW and 2.5m LT

I expect a Pokken-level performance at COMG, but it's ratio may be good. I predict 70-90k FW, and 400k LTD

100k FW, 550k LTD