I was given an explanation recently that sounded plausible - from someone who repairs diesels for a living. The ether (I think it is) in easystart removes the oil on the cylinder walls, obviously in minute quantities, and over time there is excessive (compared to "normal" useage) bore wear. The compression decreases and there is poor starting as a result so more easystart is used, till it will only start on easystart.

Others may have a better explanation.