I don't think you can say a guy who took Bournemouth up three divisions and kept them in the premier league for several seasons was bad at his job because he could only keep them there so long. Management is like politics, its rare that your career doesn't end in failure.
Some managers can improve on the job. Fergie was good at this - taking the tactical education he got from the Champions League to stay ahead of the the other Premier League Managers which worked brilliantly until the Premier League started importing managers from Champions League clubs.
Others deliberately take time away for self-improvement. Guardiola is the highest profile example of this but its the same principle if a local youth club coach decides to step back for a season to do a UEFA badge.
Good, finally we are on common ground. "Eddie Howe was not good enough to keep a club in the Premier League, but after a year on the dole he will have thought about it a lot so maybe he can now" You may be right and I now see why he was the obvious choice for NUFC. If it doesn't work out and he can't get them promoted back up, I'm sure there will be lower league clubs fighting over him in two and a half years, that must be very comforting to him, he's definitely got a good head on his shoulders at the moment.