Round 1: MHW combined vs. Splatoon 2
- A: Splatoon 2 (Splatoon's legs may eventually push it ahead by a small/moderate margin)
- B: Monster Hunter World, comfortably.
R2: MHW PS4 vs. Splatoon 2
- A: Splatoon 2, moderately.
- B: Splatoon 2, barely.
R3: MHW PS4 vs. Horizon
- Leaning towards Horizon.
R4: MHW combined vs. BotW combined
- Zelda by a decent margin.
Splatoon's growth in the west seems somewhat negligible compared to its growth in Japan. It's also likely that Splatoon 3 will come out sooner than we think. This is a multiplayer focused series, so releasing the sequel in 2 years -which could cut its legs short- isn't out of the question. On the other hand, MHW's sales won't be as heavily dependent on long term legs as Splatoon, and I don't think a full-fledged sequel is coming out until early 2021. A parallel Switch focused MH won't harm it all that much either.
My lifetime sales estimations (shipments + digital):
Splatoon: 7-9 million
MHW: 9-12 million (6.0-8.5m on PS4. Around a 13-15m total if ported to Switch)
Horizon: 7-8 million
BotW: 13-14 million
And yes @Mnementh, I'm including digital in this.