The problem with that reference to Xavi is that he is systems by himself and you build around him and that accordingly. You don't drop him into other methods of play, you optimise him and make the team play with that in mind. It's not just tiki-taka, it's possession and ball retention - anything where players comfortable in their own passing and movement give the ball to him, use him as the central hub and be more assured by him than any other in that role to get the ball back and get any kind of chain football going.
Versatility can be of merit, but first and foremost, the impression a CM leaves on his teammates and opponents is what takes precedence and where we take about dominance. Xavi does that by being the best to ever live at passing, chaining and relaying the ball, but when the other monsters are brought into the conversation, such as Neeskens (no less than 3rd between Matthaus and Rijkaard, for me) it's about their presence being an overwhelming and decisive factor not only in how a game will be played, but also how much they will oppress the opposition on both sides of the ball.
You've reduced the category to a specific type of CM, ones that are more about control on the ball over off it, which is a different discussion really, one that does elevate Modric, but also places Xavi right at the top because nobody has been able to exert as much control via use of the ball as him.
If I'm told to drop a CM into any system with any type of player and I don't know the individual qualities of the team, Modric goes in over Xavi, but the moment I'm aware it's a possession-based team, literally no CM ever gets in the team over Xavi, not even Falcao. Once you go down that route, however, Modric is up against the aforementioned plus many more whilst the only discussion happening on the other side is: who'll get to play in that team alongside Xavi?