This is a really great question. If you had a perfect conductor in air, then yes you would have no attenuation (or "insertion loss", one common term used in Signal Integrity). Real transmission lines (cables, PCB traces) use dielectric insulating layers adjacent to the conductors. These dielectrics have a property called permittivity which affects what we call "dielectric loss". Essentially, transmission line loss can be broken into conductor loss (resistance, skin effect, etc.) and dielectric loss (energy lost by causing physical rotation in the dipoles of the dielectric material).

A perfect conductor would have zero conductor loss but would still have some of this dielectric loss. I covered the concept a little bit in a webinar recently [0] but SI guru Eric Bogatin really boils it down best. In this [1] equation for loss estimation in a PCB trace, you can see the conductor and dielectric portion of loss on either side of the "plus sign". The left part is the conductor loss and is proportional to the square root of the frequency. The right part is the dielectric loss and is proportional to the frequency of the signal, as well as proportional to parts of that permittivity (Dk and Df in that equation). To answer your question though, it absolutely would allow for much longer cables since you eliminate one part of the loss.

Sorry for all the ridiculous detail, if you're still interested in the topic please feel free to reach out for more info! Contact info in my profile and at [0].

[0] https://wiki.shielddigitaldesign.com/High_Speed_Design_Wiki/...

[1] https://www.edn.com/loss-in-a-channel-rule-of-thumb-9/