To be clear, a 3.0 cable would work with a 2.0 extension cable too, just that the bottleneck is going to be that 2.0 extension cable. USB is backward compatible technology. The bottleneck will always just be the oldest revision.
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Why wouldn't it Work if it's too long
There is no reason, really. I am sure there is some sort of logistical limit to the length due to power, etc. but I would suspect that would be very long. We are talking many meters here, if anything. In short, I doubt that would become a problem for you.