I'll play devil's advocate here:
Foster's time as head coach with the Chiefs wasn't great. But people can develop and improve if they show that they can learn from past mistakes.
People defending Foster always say this, but it isn't true. Poor coaches never turn into good coaches.
By the time Foster left the Chiefs he had been coaching for well over a decade. If he wasn't good by then, he never was going to be. He showed no upward path then, yet we are meant to believe that he would start showing one twenty years into his career? That is the height of optimism.
While people do learn from past mistakes, they also carry over old ideas and ways from the past that rapidly get out of date. Once you get to a certain amount of experience, the new things you learn generally don't cancel out all the old baggage that you carry.
This has been studied in various fields. Teachers, for example, are best after eight years experience, regardless of how old they were when they started. After that time they generally get worse with added experience.
It's why one of the best things a person who is a bit jaded can do is do something else, even if only for a short while.
Foster is doing exactly what the nay-sayers predicted -- picking favourites he won't drop, having fantastic performances followed by inexplicably bad ones, fielding teams with game plans that are out of date. That's who he is, and more experience is not going to change that.
Can you persuade me by giving examples of coaches who got better after more than a dozen years on the job? They are few and far between. Even great coaches tend to fade with age, rather than get better, which is also contrary to the "you get better with experience" line.
NZ Rugby got cocky. Like the great Auckland run of the 90's, then the great Crusaders run when they persisted with Blackadder, they actually believed that their systems were more important than the people they hired. As long as the man involved knew the system, then he would get results, because the system was so awesome. Someone like Robertson who was obviously going to change the system was actively avoided.
Foster was hired not because he was the best coach, and they knew that, but because he would continue the system. Turns out that their system always depended on having the best coaches after all.
So, as we saw with Blackadder and the Crusaders, a slip in results did not lead to a change in coach. They simply knew the system would win out, so they persisted for eight years of mediocrity. Luckily for them the next coach was from the system too, but good, so they started winning again. They are going to be baffled in Crusader land when the system doesn't work when he goes. The Blues were less lucky. They could not get their heads around why their system didn't give them championships once Graham Henry left. It turns out their much vaunted system required having the best coach, after all. (They didn't even get it once Henry returned and gave them one more championship, they were that pig-headed about it.)
The Chiefs persisted with eight years of Foster. The Crusaders with eight years of Blackadder. The Blues decades of uselessness. That NZ Rugby will come to their senses and realise that the coach is the problem is still some four years away. If we are lucky.