While i do agree Monster Hunter World will easy sell over 10 million, hell, actually i do believe it could sell over 12 million, most people underestimate Splatoon legs, just like usual...
Splatoon, with Mario Kart, Mario Odyssey, the next Animal Crossing will probabily be the most leggy games on Switch.
They are gonna sell millions even in 4/5 years after release. I see Splatoon 2 finish somewhere between 12-15 million.

I think it will be close but in all my pool i said Splatoon should win, however, would not surprise me if Monster Hunter World outsell Splatoon worldwide... it depend on MHW legs and if PC version is a hit. Right now i see the possibility for MHW to sell even over 15 million, just... i don't think it will. But it has a chance.

Zelda, while so far had insane legs, i feel this won't last too long... not like others Nintendo games. All considering, Zelda is a launch title, and if we watch closely, we can see how EVERY Switch games had pretty good legs in 2017, including old ports. I think that after 2018, Zelda legs won't be as impressive as others Switch games.
Doesn't change that it already sold A LOT.

This is fun because i think that all MHW, Zelda BotW and Splatoon 2 will end in the 12-15m range, if i have to choise:

Splatoon 2 > Zelda BotW > Monster Hunter World

But really can go either way

What i think is out of comparation is Horizon. Withouth bundle, there is just no way it can compete with those games. If Sony do a bundle then maaaaaaaaaaaaaaaaaaaaaaaaaaaybe, but withouth that, it won't even come close.

Round 1: MHW combined vs. Splatoon 2

- A: Japan (MHW is PS4 exclusive in Japan). Splatoon 2 will win long term Japan.

- B: Worldwide. Combined sales of all platforms would be enough to beat SP2

R2: MHW PS4 vs. Splatoon 2

- A: Worldwide. Splatoon 2 will win World Wide at the end of the day due to legs

- B: Worldwide (Excluding Japan). Exclude Japan and MHW wins.

R3: MHW PS4 vs. Horizon. MHW has this won easily long term.

R4: MHW combined vs. BotW combined. This one will be close, but I'll give it to BOTW.

PS4/Xbox games are usually very front loaded on sales. Monster Hunter might leg it in Japan but I expect western sales to slow much quicker. Nintendo games are renowned for having legs so im very intrigued to see how Monster Hunter and Splatoon face off.

R1 - MHW is killing it in Japan, pushes MHW above both in A and B

R2 - Splatoon 2 currently at 5M+, we dont know just yet what numbers are for the PS4 vs Xbox but obviously heavily leaned towards the PS4 userbase but I do think Splatoon 2 will have better legs worldwide compared to MHW.

R3  - MHW should already be ahead of Horizon? 

R4 - Zelda will go on to sell over 10M, I cant see MHW do that.

Good thing Capcom made this game on PS4/Xbone/Pc. Without the expansion is at almost 16 million units and with the expansion around 21 millions. Combined, PS4 version should be around 10 million units. 
Huge success for Capcom.

Capcom included master edition(Base+Iceborne) in base MHW sales number, only iceborne is tracking separately since you can't play MHW iceborne without the base version. The same reason why master pass of BOTW wasn't included in total number of Zelda BOTW because it would be stupid lol.

Yup.

3.5-4 million in Japan alone while being almost twice the price.

Record breaking revenue and profits for Capcom FY.

11-12 million overseas.

Outsold every single 3DS entry in one go in almost a 1/4 of the time.

Best selling third party Japanese game of ALL TIME.

MHW and Iceborne are still selling. I expect LTD to be 25 million.

Iirce MHW reached 10 million before the PC release and the western split was like 80:20 on PS:XB. That's 8.8 mil on PS4 and 1.2 mil of XB1. I expect it be:

PS4 : 10 million, XB1: 1.5 million, PC: 4.5 million.

Two things. 3 different platforms and uber-expensive development. and wrong numbers. PC maybe 6,5-7 and PS4 8 millions. Steam spy estimates on STEAM between 5-10 M. U don't have switch counterpoint because of that you argument go down. Capcom maybe lost ton of money not released a Monster Hunter Switch. 

25M is impossible take a look at Zelda BOTW leg vs MHW unless next gen SKU were released it as well. 

We now know that it turned out like this:

Round 1: MHW combined vs. Splatoon 2
- A: MHW
- B: MHW

R2: MHW PS4 vs. Splatoon 2
- A: MHW
- B: MHW

R3: MHW PS4 vs. Horizon (HZD? or Animal crossing?)
- A: Animal Crossing. (if you ment HZD, then MHW)

R4: MHW combined vs. BotW combined
- A: BotW

Monster Hunter World did surpriseingly well.... its gone over 16m units, and over 5m of the expansion.