A more demanding service can highlight defects in the cable pair.
The important point to remember is that xDSL technology uses the cable pair as a radio frequency transmission line. So thinking about the circuit in terms of low voltage DC or AC (at 50 Hz) would be quite wrong.
Let's make the assumption that a typical pair, in a typical cable, has, over the last 17 - 18 years, carried each category of xDSL service as soon as it became available. The upper frequency that the pair has been required to carry has steadily increased. In the beginning was G.992.1 (fmax 1.1 MHz), then G.992.5 (fmax 2.2 MHz), G.993.2 Profile 8a (fmax 8.5 MHz), G.993.2 Profile 17a (fmax 17.6 MHz) and finally G.9700/G.9701 (fmax 106 MHz, with up to fmax 212 MHz proposed). As the frequency increases so does the "skin effect". With ever higher frequencies being propagated along the pair, lesser and lesser of the metallic pathway is being used -- the current is being constrained to an ever decreasing annular ring, hence the descriptive phrase "skin effect". A less than perfect pair may have been adequate for telephony and, say, G.992.5 (ADSL2+) services but could be significantly disruptive to a G.9700/G.9701 (G.Fast) service.
I include the use of aluminium or aluminium-alloy conductors within my phrase "less than perfect pair". Aluminium is a highly reactive element which passivates rapidly in the presence of moisture and oxygen to form an amphoteric oxide. Aluminium oxide is far less conductive than the pure metal. Hence a cable pair made of an aluminium-alloy will have an outer layer (a "skin") of less conductive aluminium oxide. Which would be a big "no" when attempting to pass a high radio frequency current over such a conductor.