A company that puts out a console may offer a cheap upgrade route for it, if it is their IP.  Titanfall belongs to EA, that just wants to sell games, NOT looking to promote a platform.  

As far as a console bundle goes, you would need Microsoft to moneyhat yet again, because they don't own the IP.  And also, where do you get the idea they will market it like Halo?  Titanfall isn't locked to be an exclusive and is not published by Microsoft, the way Gears or Halo was.  The thing is that it is NOT Microsoft's IP, is not exclusive just to the ONE, and EA would have every interest in having it go multiplatform, if possible, just like they did with Mass Effect. 

How long does multiplayer only Titanfall keep up sales, if it gets a sequel?  The community then moves to the sequel.  At that point, the sequel is also likely on the PS4 to, and getting hyped up there.  

And I was asking lifetime between just the XBOX ONE version vs Shadowfall on the PS4.

How does releasing Titalfall on other platforms besides the XBOX ONE cause it to get better sales on the XBOX ONE?

I wonder how we would be able to tell if a majority of the buyers of each game purchase it digitally...

That's a strange comparison, Titanfall is being released for three different platforms and you're only counting one?
Killzone is obviously going to win in that case since people that want Titanfall have 2 other options that have a bigger install base. Unless the Xbox One really picks up momentum Killzone will be the clear winner.

The comparison is trying to look partly at the issue of Titanfall being said to be an XBOX ONE killer app.  Because of this, I wanted to focus just on the XBOX ONE.  What you said was one of the reasons I was thinking.  However, as part of the motivation for the post, despite the multiplatform angle, and it being multiplayer only AND it being a new IP, people are still arguing Titanfall is a killer app that will sell tons on the XBOX ONE.  Because of this, I wanted to have people predict how well it would do against the PS4's closest rival to it, Shadowfall.  And this is what I wanted to see.

Also, check the poll.  I would try to factor in people misreading the poll, but more people on here are figuring Titanfall will outsell Shadowfall.  I am not sure how much is people thinking on all 3 platforms.

Activision has already annouced that they will offer $10 upgrades for Call of Duty: Ghosts.  Sony (Andrew House) announced at Gamescom that a number of games will be available for the "limited time" offer, including Call of Duty: Ghosts, Assassin's Creed 4: Black Flag, Watch Dogs and Battlefield 4. Activision, Ubisoft, EA and Warner Bros. were noted as publishers partnering with Sony on the upgrade initiative.  I fully expect that EA will do this with Titanfall.  It allows them to get gamers to double-dip on purchases anyway, even if it is only at a reduced rate.

http://www.joystiq.com/2013/08/20/sony-to-offer-discounted-upgrades-to-ps4-software-on-select-ti/

There were many XBox 360 bundles including 3rd party games.  Specifically, this Call of Duty: MW3 bundle comes to mind:

http://www.gamestop.com/xbox-360/consoles/xbox-360-limited-edition-call-of-duty-modern-warfare-3-console/93193

With Titanfall already being hyped by the gaming media as a system seller, it only makes sense that Microsoft would try to capitalize on this and market it even more.  I think it would be foolish of them not to.

???

titanfall