Well, we already have one example of a failed Reconquista: Anatolia.Aris Khandr said:I would love to see an alternate history story about the outcome of a failed Reconquista. Had the Muslims not been ejected from Spain, and thus no Columbus or Spanish Inquisition, how would the modern world look today? Would Islam be less prone to extremism? How would a strong Muslim state have affected British and French colonialism post-WWI? Would there even be a state of Israel now? What would be the status of the "New World"?
The Spanish Inquisition wasn't nearly as impressive as we imagine it to be, so it's absence probably wouldn't matter. Columbus, likewise, isn't that important and is interchangeable with any number of other ship captains.
Islam's decline into extremism is a reaction to stress from outside forces and - more recently - magnified an infusion of oil money. If Europe never achieved ascendancy, Islam would have little reason to develop it's modern day inferiority complex.
The flipside is that Christianity's inferiority complex - the one that spurred it to desperate trans-oceanic graspings at straws in the first place - would be intensified. Maybe it's too big a gap for real ocean-going ships to be developed to reach the Americas from France or Britain without knowing that there's land there to reach. Historic colonialism and the ultimate victory of Europe wouldn't be impossible without Spain. But without Colombus and a secular answer to encroaching Islamic armies, Europe may have just kept doubling-down on ever-more-fervent religiosity.
In the latter case, there is no French or British Colonialism (except possibly when the Turks arrive) or a recognizable World War I. The Muslims are the main cast of history in this part of the world now, leaving Christians to play in the corner.
Should Europe have triumphed, I can easily see Spain becoming (again) the stomping ground for Napoleanic armies and treated by Europe like Tunisia or Egypt. Assuming it stays independent, it may adopt a neutral position towards the Christian fraticide of the First and Second World Wars.
Or it could have modernized and become a solid player in world politics. I don't see this as so likely, given that an Islamic Spain would feel culturally and religiously drawn into the politics of the Middle East rather than those of Europe.