Why do people think just because a Flowering Pear tree (Bradford or Aristocrat) is 15 years old, that you should cut it down & replace it?

all the internet sites that are against Bradford Pear trees particularly, some even referring to them as an "invasive weed tree" that is harming the environment, etc. I have read they live on average 15-25 years. and you can make them live longer by butchering the CRAP out of them and make them look ugly, but really If you have an ARISTOCRAT PEAR tree,,,is that the case? wont these live to be 50 years old even? and not even need to LOP THE CRAP OUT OF THEM?