Potential Difference is similar to Temperature Difference in Thermodyanamics. When we talk about potential by convention we consider the potential at infinity to be zero. So you are right when you say that the potential of the surface of sphere is KQ/R. But the potential of point b is not KQ/X. The potential of ground is considered to be 0 since it is considered that ground is neutral. In your case the end points are the surface of sphere and a point on wire just above the surface of ground. But on the ground the potential is 0. So as we go closer closer to the surface of ground,the current in charge would reduce but between the end of the wire and ground potential difference still exists therefore ,charge will still flow. I hope it helps!!
Karthik Nambiar
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