> The Shannon limit is constant if you assume reasonable but idealized SNR values for the medium
The Shannon limit is changed by changing technology over the medium, not the other way around.
If you signal with light on/off pulses, you get one limit. If you add polarization tricks (using different physical properties and tech), you get another limit. As you add QAM and a zillion other tricks, you get another channel limit. If you add quantum superdense coding, you get another channel limit. Each of those, until we learned there is yet another layer of physics and tech, would be "the Shannon Limit." All of these can be done on the same medium.
The Shannon limit is a mathematical *model* of a channel. It's not a physical/technological limit.
Here [1], for example, is a paper pointing this out for transoceanic undersea optical cables. "As pointed out in Section 9.3, the Shannon limit is only limiting if we assume there is no technical way to further improve the QoT..."
Technology changes routinely change the "Shannon Limit," since that limit has almost nothing to do with physics. Physics and the signaling technology define a Shannon Limit for that particular channel combination, nothing more.
[1] https://www.sciencedirect.com/topics/engineering/shannon-lim...