Depends on total sales. If PS4 sells 140 million and Xb one sells 100 million then that 40 million gap really doesn't mean much because Xb one (and brand Xbox) will still have a huge install base and good long term prospects (assuming we get a 9th generation) that any 3rd party would be foolish to look at exclusivity simply on the basis of sales potential.
However if PS4 only sells 70 million and Xb one only sells 30 million then that 40 million gap is substantial.
Personally I think PS4 is likely to sell between 80 and 100 million and Xb one between 60 and 80 million.
And Wii U between 30 and 50 million.
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