So you mean that each year after 2015 has a 50% chance of Kessler syndrome occurring?
Given the rate at which stuff goes up compared to the rate it comes down again, ITIRC that the total volume of junk hits the 50% probability of the syndrome occuring in 2015. Each year after that there is more junk and only the same amount of space for it to be in.
There have already been a couple of collisions, but luckily no cascade has developed - so far. It really is only a matter of time though if we keep sending stuff up there at the rate we are.
I wouldn't be at all surprised if Sentinel1 doesn't last long enough to see an essentially ice free Arctic summer, and I'm not talking about an ice recovery either!