I disagree.
Whilst you're correct in pointing out the payload advantage of the IL-78, you fail to mention the host of negatives that come with operating a jet transport of that size, some of which I'll bullet point below:
-Despite it's excellent landing gear with multiple bogies, the IL-78 has a higher PCN (pavement classification number), meaning the landing surface has to be a certain strength. The C-27/C-130 are pretty much unrestricted in that regard.
-Turboprop aircraft are much more fuel efficient than jet transports, especially between props and russian jets! Makes a difference when your defence budget is stretched. Also note, the props on a C-130 are much more reliable, have a greater TBO and are cheaper to repair and/or acquire due to the huge spares market.
-Turboprop aircraft have better acceleration and STOL performance compared to jets. This is an especially important capability given Pakistan's varying terrain and past requirements needing to operate out of unprepared strips.
Having said that, the IL-78s were obviously needed for the jf-17, spada and grifo transport requirements.
I just hope that given the vintage nature of Pakistan's C-130 fleet and the types replacement with the C-17 and A400M in Western forces that the PAF manages to acquire some bargains!