This depends heavily on what type and where the diseased or decaying wood is. It can in some cases be a fire risk. It can be a safety risk if close to people and animals but also infrastructure. It may also be either a dormitory for undesirable insects and or diseases. Many of these can even be invasive. Professional evaluation should taken and not just make an assumption that nature will benefit. No offence intended to you. The current plant passports in the UK and Europe are hoping to combat tree diseases and prevent them from spreading. It is currently an offence to move Ash saplings and there is a quarantine for the movement of pines in sussex/Kent. Previous attempts to quash diseases like Dutch elm were a catastrophe Thanks to poor control and failures even from local authorities.

I think he meant sad from the point of view of it not being turned into useful boards in the sawmill.

I too, am an Indiana, southern, native. In our part, the Sycamore trees tend to start center rotting early in their mid-life, yet continue to grow. There are stories told down here in Lincoln country of immigrating families living inside of these hollow trees while the cabin was being built.