It's absolutely a physical limitation and to argue otherwise is a waste of time. You can use complex multiplexing algorithms to squeeze more out of a single wireless channel, but at the end of the day you're sharing that airspace with anything from another WiFi signal to a microwave oven. To go faster with wired all you have to do is, like you said, add another pair.

EMI can be sheilded, yes, or you can move to optical and then you're literally transferring at the speed of light on a dedicated medium. You simply can't do that with radio. It's not physically possible unless there is some signaling technological breakthrough that we have not yet conceived.