I think there's a lot of overly optimistic assessments of John's Syrian campaign going on here, both in terms of its goals and its accomplishments.
Firstly, consider the context. Syria c. 975 is rather a mess. The Fatimids are new on the scene and still trying to consolidate their presence in Syria, and in the previous year they had to deal with a Qarmatian invasion. The loyalty of Fatimid clients and allies in the region was questionable. Damascus had been taken from its Fatimid garrison by Alp Takin (or Aftakin) earlier in 975, who was a former soldier of the Buyids who then became a Qarmatian ally and afterwards alternated between fighting with and fighting for the Fatimids. In other words, John was taking advantage of a land in considerable turmoil with a lot of internal divisions and weak rulers who were ready to be tributaries to the strongest power.
Indeed, making tributaries seems to be what John was after. Rather than an being existential threat to the Fatimid Caliphate, John's Syrian campaign was essentially a pacifying expedition, intended to take plunder, gather tribute, and prop up client states in Syria that could be a useful buffer. Sidon, Nisibis, and Beirut paid tribute (among others), while according to a Damascene chronicler Alp Takin was able to buy off the emperor with gifts, perhaps making a nominal submission. No significant pitched battles were fought. This is "conquest" only in the loosest and most ephemeral sense; while it's possible that the campaign of 975 could have "prepared the ground" for a later and more thorough advance of the empire's borders, his actual campaign was in the classic imperial mold of asserting dominance beyond one's own borders by knocking some heads, taking some tribute, and supporting a few favorable clients. It's only in the 12th century account of Matthew of Edessa, writing centuries later and in the age of the Crusades, that John is credited with marching all the way to Galilee and given a religious motivation of trying to liberate Jerusalem, thus portraying a rather more restrained (though successful) tribute-gathering raid as a sort of proto-Crusade.
Yes, John Tzimiskes was a good general, and yes, it is plausible that he could have done more in Syria than he did. But to treat the 975 campaign as an abortive "Byzantine Reconquista" that could have seen the empire restored to its Heraclian borders given a few more years of John's life is totally unwarranted. Permanently welding Syria to the empire was probably not even John's intent, let alone Palestine or Egypt. They were, I would argue, beyond his capability to take and certainly to hold. The story of Alp Takin demonstrates just how changeable the loyalties of these Syrian clients were, and John's one-time Turkish and Qarmatian "allies" were hardly going to remain in his camp for long. They were, for the most part, friendly as long as the Byzantine army was nearby and no longer.
The best case for John, IMO, is a sort of "Syrian Hegemony" - Syrian client rulers are kept friendly by the threat of force, tribute flows to Constantinople with the "encouragement" of the occasional imperial expedition, and the Byzantines have a nice broad swath of protectorates to pad their frontiers. Perhaps some coastal Syrian cities are brought under more "direct" rule and placed under Christian (that is, Greek/Armenian) governors; perhaps a few new "ducates" are erected, like for Tripoli and Beirut just as he set up doukes in Antioch, Chaldea, and [Byzantine] Mesopotamia. Perhaps Jerusalem even becomes a tributary and John visits the city for a real propaganda coup. The Fatimids are contained by this cordon of client states; maybe they even kick in some tribute. All this, however, probably requires everything else to go really, really right, and if there's one thing Byzantine history teaches us it's that empires are rarely left alone by external and internal events to carry out their ideal long-term foreign policies. Let's not forget the Bulgarians are still kicking and Basil II didn't fully crush them until 1018, long after John's death even if we generously give him another 20 years of life.