You know that the only mainline mgs game thats really exclusive to a sony platform is mgs4 right?

I really dont see the problem.  Assuming a fanbase of 5m for mgs4 that bought a ps3 to play it, what difference does it make if mgs5 sell 5 m on multiple platform.  Some fan just change platform because they had the chance to do so.  That said, mgs 5 sales is better than we thought because digital sales is bigger now compare to mgs4.  Digital sales in US is 15k at the minimum based on the ratings, now add digital sales from japan and europe for every platform and that should tell you that sales are better than we think.

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Why do ppl assume multiplat = more sales?

Your fanbase is your fanbase, whether thats spread across one console or a bunch, the fanbase doesnt necessarily grow with the amount of consoles its availalbe on

Made some edits to make your point clear in the OP.

Amazing how this turned into an exclusive vs non exclusive.

But while we are at it, let's not forget that the PS3 was DYING for an exclusive game and MGS 4 was like a fresh glass of water for the PS3 and SONY that were going through hell after the bombs that were Haze and Lair

In addition, a lot of the great games, that are multiplatform now, were exclusive to the 360. Bioshock, Mass Effect and Ninja Gaiden just off the top of my head.

Underwhelming, why? More platforms doesn't necessarily means more sales (given your theorical case, it would actually mean the fanbase has expanded, which means it is less underwhelming, actually), nor the fact that one platform substract sales from the other. The Phantom Pain had an amazing marketing, and stellar reviews all across, plus tons of controversies that also pushed the game forward. Its sales are what they are, they're no product of the game being multiplatform, but the reasons I posted earlier.

And even then we're ignoring whichever legs this could have. For all we know...

The digital sales were probably huge especially when you factor in STEAM too.

Should have been PS4 exclusive with that LE console bundle made for the U.S. too. PS4 fanbase is big enough to support the game. XB1 players aint buying it base off NPD numbers of only 180K.

Sometimes being exclusive is better for a franchise.

Thing is, the only exclusive game of this franchise is MGS4. Metal Gear started on a Microsoft system, followed by Sony consoles and then it also went to PC and Nintendo platforms.