While I agree with blaargh and MadFan23 that the players have had chances my opinion is that 13 is overkill. And publishing that such a list exists will lead to much more than 13 players thinking they do not have the coaches behind them.
And as for the last two years, well I don't really think 2016 can be looked at as a year of expectations. There were disappointing losses but except for the Carlton and Fremantle games not unexpected ones. We started off on a back foot with WADA and injuries just made it worse. I'm not sure if we put an unchanged side on the field at any time, if we did it wasn't often.
Thing is, there are 13 (probably more) players who are absolutely tradeable. Of course, not all of them will be traded. What this is designed to do is send a message - to put them on notice that they are expected to perform to a high level on a consistent basis. They'll respond either of two ways:
1. It could be the making of them as men and footballers, and lead to a tough, successful culture in the playing group; or
2. They fall apart under the pressure, in which case the club will go for a full rebuild, and build the playing group it needs for success.
KT was right to say that the players need to decide whether they're able to meet the standards the club sets. Allowing it to become known that 13 players are considered tradeable is partly designed to get the players thinking about that question.