So it confirms that fact that nothing was left at Mianwali AFB ....PGs gone, L-15 in...
Not necessarily. The PAF wanted to get a new LIFT since 2017-2018.
I'd rather say the real need now is that the majority -- if not near-entirety -- of the PAF fighter fleet will comprise of 4/4+-gen multirole fighters in a couple of years. Consequently, every fresh GDP will have to fly either a JF-17, J-10CE, or F-16, thus exposing inexperienced people to comparatively complex and capable systems vs. F-7P/PGs and non-upgraded Mirage III/5. The old F-7s and Mirages don't have TDLs, multimode radars capable of BVR, targeting pods, HMD/S or HMCS, ECM, SOWs, etc.
Earlier, I think the PAF was planning to train new pilots with everything they needed to know at the OCU level. In fact, not only did they raise a full OCU unit for the JF-17 via the JF-17B, but they also planned to provide each squadron with 1-2 JF-17Bs to continue training. So, there was a plan to get new pilots up to speed at two levels -- i.e., OCU and the frontline squadron.
However, after doing this for a couple of years, I think the fighter units sent feedback to AHQ about the need to provide more training at an earlier level, hence the need for a dedicated LIFT. I can see the logic: You have all this great equipment, but many of your aircrews don't have enough training to use most of it. So, you're sticking squadrons who should be focusing on familiarizing pilots with a specific plane to also work on other stuff, which slows those units down from having enough combat-ready flights.
In the lead up to pursuing the L-15, the PAF set up a new LIFT squadron and added it to the training regimen, placing it between FCU and OCU. This was to likely develop the processes so that when the day comes, adding something like the L-15 would be relatively smooth. It didn't happen earlier because, well, Pakistan doesn't have a lot of money to work with.