Hello, I hope the following assists you.
As the clutch was slipping this will have caused the premature wear of the clutch assembly, the overheating occurs as a result of the friction heat build up as the plates slip. Under normal conditions the clutch assembly will last well in excess of 1000 miles and ridden in a normal manner 20000 miles wouldn't be unusual.
As you reported it at the service it seems a little unfair to expect you to pay for the replacement but having said this the dealer may perhaps ask why you didn't go to them when you first experienced the slipping, they may also claim that the adjustment was incorrect when done prior to the service as it wasn't done by a Kawasaki technician.
This is really going to come down to what you can agree on with the dealer, the clutch hasn't failed due to a manufacturing defect which is why they aren't willing to do it under manufacturers warranty but you have the case that the clutch was slipping from factory so the bike wasn't prepared correctly before sale.
As I said earlier it's going to be a case of coming to some agreement with the dealer.
I hope this helps but let me know if I can assist further.