B-52 (1955– 56) and the KC-135 Stratotanker (1956–57) both are still operational and will continue to be so because there is no replacement. Same case for Mirages in strike, they are still needed for certain tasks which JF-17 will take over gradually.Good News for the Mashaak indeed but why make such a major investment for the antiquated Mirages that are beyond their scrap life right now?
When you rebuild a aircraft the most expensive portion is the man hours but since PAF has the staff already on payroll its not a issue. Secondly as OEM no longer makes components for these and there is extensive infrastructure, modernizing them might be more viable and cheaper than simple rebuilds. PAF will likely keep a few of these as specialist strike platforms for a while but they are no longer the work horses they were once with the PAF.