VDSL2 VPlus/35b is not 17 MHz, but rather 35 MHz. We currently use 17a, not 35b, and VDSL2 VPlus does not exist in 17a variant, but can co-exist with pre-existing 17a deployments.
In reference to ADSL and VDSL overlay, Power Spectral Density (PSD) masks are in place to "protect" ADSL signals from being impacted from VDSL. Of course, this impacts VDSL performance (and sadly, I don't know by how much).
The starting frequency of G.fast is actually 19 MHz in Openreach deployments, and uses the range 19-106 MHz currently. As you are aware, this is to reduce the impact of G.fast on VDSL 17a.
VDSL 35b won't be rolled out because the density is less compared to 17a and because it would probably cost a lot more money than worth at this point as I cannot remember if vendors have compatibility with 35b profile with the current hardware rolled out across the UK.
If it could use all the frequencies, VDSL would never perform worse than ADSL, and G.Fast would always be at least as fast as VDSL, etc.
Aggressively curtailing older technologies would be the best case for increasing speeds/freeing up bandwidth among newer technologies that use radio frequencies. It is essentially the same premise for mobile networks, TV and radio too as what happened with the digital switchover, adjustments in the TV broadcasting frequencies for 4G, etc.
As you said previously, providers have investments in ADSL2+ technologies. Sky and TalkTalk in particular have a huge LLU presence. This was a lot of money invested into their network. Phasing out technologies is not just a problem of moving people over to the newer technologies, but is also a financial issue.
Edited by j0el (Thu 04-Oct-18 00:06:00)