Point taken, thank-you for the link to the reference (NICC ND 1405 V3.1.2 -- Guidelines on the Use of DSL Transmission Systems in the BT Access Network), and yes, crosstalk is an issue. I'll say one word - vectoring.
This is an interesting read:
and a Belgacom page (in English)
It would seem that vectoring effectively eliminates the crosstalk problem and would allow upstream capacities to be increased.
Even if dynamic allocation of frequencies to upstream and downstream use is not possible, I would still like to see greater upstream capacity available, even if this had to be fixed in advance.
I can also dream of CuPON, although the likelihood of its being implemented is vanishingly small.
CuPON: The Copper Alternative to PON 100 Gb/s DSL Networks