I won't walk away from a franchise I love. Tomb Raider is the only game franchise I follow, so for selfish reasons, I'll stick by it and it may mean I'll have to save for an Xbox one in time. I don't see any reason why SE/CD would not put the game on the PS/PC platforms once the 'duration' is over, but should that happen, that would be a crying shame for a large part of the community. But, I love Tomb Raider, and all the games to date are still there to be enjoyed, I enjoy being a part of this community so I'll stick around to be a part of that.
One thing to note though: TR'13 had so much emphasis on Lara which is continuing on in RotTR and beyond from what I gather. TR'13 had an audience who became invested in this new Lara and her story. Although the same number of (eventual) sales aren't guaranteed, I imagine that CD would want their audience to stay with Lara and continue to enjoy her adventures. With that in mind I find it hard to believe CD/SE would cut off that opportunity to sustain that player/protagonist relationship they were tying to build with TR'13.