The fact is that the high cost of campaigns and the advantages of incumbency are so great that it is very rare that a sitting incumbent faces serious opposition -- if they face any at all! And keep in mind that if an incumbent runs unopposed, the election is simply cancelled.
Where there are no term limits to mandate open competitive elections at least once every eight years, it is quite likely no competitive elections are being held. The voters do not even get an opportunity to weigh in on their representatives.
Case in point: 26-year veteran commissioner Karen Marcus last faced a general election challenger in 1992, nearly 20 years ago. Yes, 20 years!
For the regular competitive elections and rotation in office that healthy democracy requires, elections need to be complemented by term limits.