when PAF got Mirages originally what was their role?
PAF got into a habit of inducting Multirole aircrafts through F-86 Sabres. F-86 successfully shot down aircrafts and also conducted strike missions. F-104, a rocket with wings, was considered more of a air superiority aircraft and didn't suit any role eventually.
Mirage III was inducted in PAF as Multirole aircraft. The same pattern continued with F-16 in 1980's and now with JF-17. It also had a recce variant, atleast 3 recon versions were inducted. Before the Mirage III recce version, T-33 was tasked for recon missions. Those who say that Mirage-III has the speed should know that primarily T-33 lacked speed to fly in IAF's dense AD network therefore Mirage III recon version was deemed necessary for this task. Interestingly, PAF made F-86 night intercept capable and this was also considered a requirement for Mirage-III from the onset. PAF's pattern has been same in many areas of aircraft induction. Just like single seat induction of JF-17, PAF went for single seat Mirage III and inducted a squadron 40+ years ago. Afterwards 3 two-seat trainer versions were ordered. The single seat version was made all weather and night missions capable for strike missions. In 1971 war, Mirage III conducted air defense and took part in strike missions as well as Recon missions. They were equipped with Aim-9 and few French R-350. Aim-9 was considered in high esteem by PAF, so Mirage III compatibility to Aim-9 was of utmost priority.
did it change to dedicated strike role later?
Mirage-III was envisioned as a fighter and used as such both by Israelis and then PAF. Mirage-III evolved into Mirage-V which was considered a strike oriented aircraft. Mirage-V has an extended nose, said to have two extra pylons. For ease of understanding, basically F-16 Block 15 evolving into Block-40 and above with strike role in mind. Therefore Mirage-V was tested and inducted with a variety of air to surface weaponry (bombs, missiles, nuclear weapons), including naval support weaponry.
If you look at Project ROSE, Mirage-III as upgraded as an air defense fighter and Mirage-V was upgraded as a strike fighter with excellent night strike capability.
what made it so special?
For PAF, Mirage-III was a step up than F-86. The Sabre had bore the brunt of 1965 war in all roles. B-57's were bombers and F-104 had little to offer. Just like today, the way JF-17 compliments F-16, the same way Mirage-III complimented F-86 Sabre. While the USA put sanctions in 1965 war, there was a requirement for another non-US aircraft. In 1960's and 1970's, it was the Mirage-III and today its JF-17.
Secondly, Mirage-III could be upgraded and carry a variety of weaponry, US and Non-US ordnance. This made the aircraft very versatile and flexible for upgrades. Ra'ad ALCM is just one example after 2000's.
Thirdly,ROSE upgrade is a very budget efficient upgrade that 68 x Mirages have gone through, even being a French aircraft. Upgrading same number of F-16's did cost billions of USD.
Fourthly, one of the few aircraft of its era to conduct terrain hugging flights.
what other Airforces got them for multi role or strike role only?
Israelis got Mirage-III for Air defense role, but then invested in Strike version Kfir . Mirage III strike payload was moderate, but due to its multirole capability, it was used as bomber by Israelis. In later wars, Mirage III was primarily used in air defense role. Argentinians had difficult time using Mirage-III in strike role and shifted them to air defense role. South Africans resorted to use Mirage-III for air defense roles also which later evolved into Cheetah version.
Mirage-V was a different story. It's kfir variant was extensively used by Israelis for strike missions, even before and after induction of F-16's. Another variant Nesher was also flown by israelis in air defense missions and performed extremely well.
why Mirage is fit for a strike role better than JF-17 or F-16?
F-16 is better suited for strike role. The embargo saga of F-16's made Mirage the primary strike fighter for PAF. Had PAF acquired 111 F-16 A/B's and follow up orders of C/D versions, the story of Mirage III and Mirage V would have ended at some point in PAF and not dragged on till today. 40 decent F-16's and above 100 Mirages with few upgraded ones was all that PAF had in 1990's.
Mirages were also the first aircraft to get BVR capability in PAF. Having said that, Mirage-III and V both could carry as assortment of weaponry as stated earlier, which gave PAF flexibility to conduct strike missions. The upgrades for F-16 Block 15's had a bigger cost, still PAF made them capable to fire AGM-65, carry ALTIS II targeting pods and possibility to fire AGM-84 Harpoon apart from legacy Mark-82/84 bombs.
what will be a true representative of Mirage then? a modified JF-17 with some compromises and some additions?
A follow up true representative of Mirage-III would be Mirage-2000 and then probably Rafale.
JF-17 has limitations in design which can be overcome with structural changes, if PAF is interested.
would future blocks of JF-17 ever be able to match its its capability that comes with speed?
Speed is just one side of the game. Electronics (data link+controlling UAV, EW, radar, sensors), ability to carry all types of weapons (including AAM's, ALCM's,Missiles, Bombs) along with stealthy features (design + internal payload too) as well as reduced RCS is the future.
or like for like replacement of Delta wing jet like Grippen or J-10? Mirage 2000 (their production stopped their spares maybe an issue)
PAF interest has been in many planes, just like window shopping. Gripen was evaluated and deemed a certain possibility. J-10 was almost ordered. Similar story with Mirage-2000 in 1990's continuing up til now. Its not about replacing a delta with delta, PAF has to induct an aircraft which suits its requirements. So far PAF is only interested in inducting JF-17's (newer blocks) and F-16's (any block). In aviation industry, the future is stealth, not delta really. This is why project Azm has become a part of PAF.