How to prevent tree stump from decaying
Where I live there are a lot of old trees (often landmarks) protected by laws as natural monuments. Rotting of tree bases is a common phenomenon while the living parts are saved by a multitude of measures.

Bases are prone to rotting due to moisture and work of fungus and insects. The dead wood is often removed as a measure, so the remaining trunk is hollow. This reduces the overall weight of the trunk, but it has to be stabilized afterwards. This is sometimes done by steel cables connected to ground anchors. 

If that is not sufficient, sometimes the hollow trunk is filled with concrete after applying protective substances onto the remaining wood. Additional water draining systems have to be used to prevent further rotting.

All these measures usually can only delay the decay for some time.