It may have been only one season but we were playing our best football in decades under Martines. You can't take that away from him.
It appears to me that at some stage, and we don't know exactly why, but the senior players and maybe some of the staff weren't having him anymore. The enthusiasm died off and the passing football became slow and predictable.
You are right. His first season was excellent and some of the football was great but why would the players switch off and refuse to play that way again or refuse to do what he wanted. Maybe it was over complication, maybe a poor run and they seen RM didn't try to change anything. I always believe that the Chelsea defeat at home (6-3) was the start of the end for him with regards to the players and the leakings in the press of the defence not being worked on for set pieces. It seemed to turn sour from then on and he let the players walk all over him and maybe down tools. They didn't buy into his methods anymore and RM let them get away with that attitude. I know we really cannot judge Koeman till next season but IMO he would ditch players with a click of his finger if they or he felt had downed tools or questioned his methods. IMO that is what any manager should do...RM didn't. One thing I have no doubt about is the team has vastly improved compared to the previous 2 seasons despite it being a bumpy road.
I suspect he was too soft with them and then when hard decisions had to be made he didn't command enough authority.
We'll have to wait and see if Koeman's being more practical and a lot more authoritarian to stop the rot and get things under control, we won't know until next season plays out.