Have to second ABIW's comments - the J was only meant to replace half the Ks - the 'short term fleet' as they were known. The remaining Ks were always meant to soldier on until finally replaced by the A400M starting around 2010. The problems with availabilty on the K fleet are not in any measure due to the late arrival of the J.
Long term life of the J fleet might well be affected by their current over usage due to the inability of the K to shoulder any of the burden.....