It's symply a matter of maths, it depends on which value the AVERAGE ratio of Wii's weekly sales to the others' combined ones will stabilize on.
Wii's market share (%) = 100*Wii avg wkly sales /(PS3 avg wkly sales + XBox 360 avg wkly sales)
Obviously, as the functions of weekly sales are neither continuous nor known thus far, we won't know the exact result until the end of all three consoles' lifecycle (*), but knowing the averages over a time span long enough we can draw some approximations and if we can determine bounds to minimum and maximum values we can also do some predictions, but only over a short period.
But what we can say now is that if actual market share is less than 50%, Wii's weekly sales must be greater than the sum of the others over a time long enough to make it possible to reach 50%.
(*) but by then the result will be more easily and trivially calculated as 100* Wii total sales / Others total sales
