Why would you assume market size will remain constant, even though it never has in the past?
Well, I don't think it will stay constant and assuming it is, doesn't mean that I expect that. This because I don't know YET which way it will go...
The market size may INCREASE because
a) Population increases
b) World becomes richer and the console makers open up to new markets
c) Some of the new casuals brought in with the Wii stays in gaming
The market size may DECREASE as well because
a) The Global recession affects many developed countries
b) A good chunk of the Wii Casuals (drawn to motion controls) leave the market entirely
c) Another portion of the casual gamers leave the market for mobiles.
All of the effects above are TRUE and IN PLACE but we don't know yet which ones are dominant. So we don't have enough information for whether or not the market will get smaller or larger; in this case, the best bet is to assume it's constant UNTILL better information is revealed. Meanwhile, I keep my estimates in a range to minimize the error. So in the future, it's unlikely to change the point estimates but rather just the ranges.
Regional Analysis (only MS and Sony Consoles)
Europe => XB1 : 23-24 % vs PS4 : 76-77%
N. America => XB1 : 49-52% vs PS4 : 48-51%
Global => XB1 : 32-34% vs PS4 : 66-68%