Militarily they might have the strength to defeat any opponent, but I don't think people are considering that other European rulers might not want them to be successful. If they tried something like this, they're going to have something of a WW2 monologue. I mean, look at the trouble when the French tried to take small parts of Italy in the same era. They had huge alliances forming against them that even included the English who were both peripheral powers and had absolutely nothing to do with Italy. There wasn't a theory of Balance of Power at this point, but there was a distinct feeling of ganging up on those who tried to dominate one region - they'd smack them back down, then let things lie, which was why in such examples, Italy being a very good one, the defeated power would simply try again a few years later (and inevitably fail). Ferdinand and Isabella aren't going to be able to take France, if they try it they're going to get in a world of trouble. They have no claim to France whatsoever, and conquering the entire country is going to be a massive effort - the English had some massively successful campaigns before this and only ever took parts of France until 1420, and that was because Henry V had a genuine claim to the crown which was recognised.
I hate to be the one to have to say this stuff, but really there's a reason that this kind of thing didn't happen in that era; this really wouldn't work, it's a monumental ask. And that's not to mention that "recreating the Roman Empire" was not in this era something that was aimed for, except for aging Papal claims of a united Christendom under Papal control...