Fisher was part of the reason the team couldn't get their crap together. The talent was mismatched to begin with and this wasn't a playoff team to start the season regardless of who coached. But Fisher accelerated the meltdown.
So, yeah he's been disastrous. Will he be as bad next year? Maybe.
His timeline is probably one half season after this. If the Knicks have Melo back + a good draft choice + a key free agent or two and they can't conceivably play for the 8th seed by the 20th game next season, Fisher will be on a timer to go on a winning streak before the All-Star game or he may get canned.
The notion that Phil will keep him around doesn't compute IMO. Fisher gets to keep $25M clams. That's a lot of salad for 18 months on the job. Nobody is going to say he was betrayed if Phil is pressured into firing him.
Now if the Knicks are losing that bad, then Phil's clock will be ticking before next season is over too. He is not untouchable. Dolan has burned up tens of millions on bad decisions and he will do it again if he feels the need to let them both go.