As a general point (not strictly on Wonders, but they are the main reason it matters), there is a global max number of trade routes a city can have. This is in a global define, and in C2C is currently set to 16. It's actually pretty easy to reach this limit, as we have tons of buildings that add trade routes, and tons of Wonders that do especially. In my current game I'm up to 12 already as I enter the Middle Ages (none of which are from Wonders in that city).
This can viewed as either a problem of it being too easy to get large numbers of trade routes (i.e. - a general building issue), OR a problem with the limit being too low. Either way though, it's easy to hit the limit and therefore have a lot of Wonders that effectively do nothing special at all.