Because of the nature of politics,most political careers do end in failure.There is endless pressure that comes from all sides.There is the pressure of raising campaign funds,and most of all from constituents,some of which can never be please.It's usually just better to work from the sidelines which ends up being more productive.
The reason why most political careers end in failure is because most politicians wind up giving in to greed and corruption. Give a person power and, too many times, regardless of intent, they begin to go a little bit crooked. It almost never fails. That's the reason why they often fail.
I think that once a politician gets elected ones and is able to get successfully re elected then he finds getting re-elected every time after that easier and easier since most people just vote for the incumbent based off of name recognition, so overall I think many political careers end in success if properly executed.
Just because no politician is going to be agreed with or approved of by every voter, that is not to say that a political career usually ends in failure. In fact in the United States most politicians retire with lots of money and prestige and a host of well paying speaking engagements.
No, most political careers do not end in failure, because some politicians manage to get reelected again and again even when they shouldn't. Ted Kennedy was reelected several times, and he left a woman to die in a river. Joe Biden keeps getting relected and he thinks "jobs" is a three-letter word. Some politicians never lose their charm.