The British built the Type 21s more ASW than AAW at a time when SSMs were being introduced worldwide. CIWS was added, but the SAMs are a bit dated and not up to todays SSMs/ASMs. The FFGs were designed for ASW missions as well, used to have a area defence AAW capabillity which is dated today. If I were to upgrade the FFGs I would add an up to date state of the art self defence AAW capability with either RAM or SeaRam... and forgo the dated area defence AAW capability...As far as i know the biggest problem with the type 21s is not the weaponry but the superstucture itself.
i think the pakisan navy is better off without the type 21
The FFGs 76-mm main gun is okay, and their ASW sonar/torpedo tubes/helicopter capability is still awesome. Add harpoons or another SSM such as C-802 forward of the bridge in racks and either RAM or SeaRam would update these used ships nicely...
When the US built the FFGs, they were facing a significant Soviet bomher threat over the Atlantic and an area defence AAW was welcomed. Today Pakistan really doesn't face an enemy long range bomber threat. A better self defence short range SAM excellent against aircraft and missiles would be better today...