It isn't exactly surprising that the longer a franchise carries on for the more prone it becomes to inconsistency because after all, there are only so many times you can put a fresh spin on an existing formula when you start reaching fourth, fifth or even sixth installments.
A lot of times, when a long-running movie series suffers from a drop in box office returns or constantly declining reactions from audiences, the decision is made to simply to hit the reset button and start all over again with a reboot. However, many of Hollywood's biggest franchises have continued on for years or even decades without being forced into making wholesale changes to the mythology.
That being said, the success ratio isn't exactly 100% and countless franchises that once marked the apex of blockbuster filmmaking found themselves being overcome by the law of diminishing returns, with many failing to arrest their descent into mediocrity.
Some franchises have aged like a fine wine and continue to improve as time goes on, while others haven't been so fortunate and ended up overstaying their welcome while refusing to adapt or move with the times, and more often than not the fans have been the ones to let them know about it.