For many cities, the goal is a healthy and sustainable urban canopy. Arboriculture has advanced a lot in the last hundred years, and we've made a lot of mistakes.

Eg, inappropriate species like Big Leaf Maple in inappropriate locations, like under power lines. Bad pruning practices. Planting monocultures (Elm). Boxing in roots.

As sick trees come down, better practices can lead to better trees for the next generation.

For some common species, 100 years is downright old, and reaching the end of their natural life anyways.