He has made changes, he just hasn't made wholesale changes. Also, there's not a whole lot you can do mid season. He's not going to complete overhaul the team's system of play mid season, and he's also not going to deviate from the systematic development of the rookies just for the heck of it.
I'm not sure how anyone can look back at this season, and how the players played, and claim that if only the coach had changed the system, or played the rookies more, we would been in the mix.
The coach is there to mold the players to the system, not the other way around. We just came from a wildly successful season last year, it would make sense for GB to expect the team to play reasonably well this year, and that it would take some time to bring the new guys up to speed. It would be unreasonable for anyone to expect GB to abandon the team's system at the first sign of adversity.
There were key young players betting more time as the season went on (Dzingle), but the rookies were brought up and down by the GM, and inserted into the line up and played in a way that was best for the development of the player, not necessarily with an eye to maximize wins. Personally I think many fans simply miss the changes made because they aren't paying attention beyond blaming the coach for team losses. To listen to Wideman speak, GB has made changes and has been coaching the team to play the way most of us want it to play. I don't think there is any argument to be made against the idea that the players let us down this year, starting with the Captain.
I expect an newish system, and refreshed players to start next season, especially with the break out of Duchene, and the chemistry with Hoffman.