I suspect there are also "spectrum management" reasons for having the limit in place.
I agree. You've given the fuller explanation of my crosstalk comment.
In time, various changes will take place that may permit higher speeds. Deployment of vectoring will help with crosstalk issues.
I suspect, too, that there will be regulatory changes favouring higher speeds.
It seems clear that the old LLU model is dying from an economic point of view. The important part of unbundling an exchange will be bringing in your own backhaul capability, which you can hook up to the FTTx infrastructure at the aggregation node. There's increasingly little point spending money on equipment to provide 'traditional' phone service and ADSL2+ when consumers will increasingly demand FTTx and voice can easily be provided over FTTx infrastructure.
The cost of rolling out FTTx means that it's only worth one provider rolling out in each location. There's little money in providing alternative 'last mile' connectivity, as shown by the virtual cessation of roll out by the original cable companies in the run-up to their commercial failure and coalescence into Virgin Media, also by Digital Region. This means SLU is only really of relevance where BT Openreach has yet to roll out FTTx. (Incidentally, one FTTC provider per location also offers the best possible scenario for VDSL2 vectoring, with all pairs in a bundle served by the same infrastructure).
If Ofcom is changed the regulatory framework to require all SLU operators to offer wholesale interconnection so there were no lines where FTTx was available but wholesale FTTx was not, the way then appears open to set a date to remove ANFP protection for exchange provided ADSL in each FTTC served area, somewhat like the abandonment of 20CN DSL products in 21CN served areas. This will require removal of ADSL from all lines served from FTTC enabled PCPs, with broadband service moving to FTTC, as the main reason behind the current protection is strong VDSL2 signals at the PCP interfering with weak ADSL signals from the exchange.
Removal of exchange provided ADSL would lower the VDSL noise floor somewhat and, more importantly, permit removal of VDSL power cut-backs put in place to protect ADSL. The removal of the power cut-backs will increase available VDSL2 speeds, especially on longer lines which are most affected by the current cut-backs on the lowest frequencies which are shared between ADSL and VDSL. If necessary, ADSL and VDSL could be served from the same point - from the PCP for lines too long to benefit from VDSL (with the corresponding increase in speeds by removing the E-side cabling from the ADSL) or from the exchange for 'exchange only' lines - though with no power cut-back, VDSL may perform at least as well as ADSL, making ADSL redundant.
The pay back in abandoning exchange ADSL in VDSL2 served areas is clear - there's no need to offer duplicate access methods, and all legacy ATM backhaul can be abandoned. This should permit wholesale port costs for an ADSL replacement VDSL2 service to match existing ADSL port costs. I'm sure 'wires only' VDSL2 will have arrived long before this happens, so ISPs will be able to prepare customers for their upgrade by sending out a new router and filters. Indeed, many of these upgrades will likely happen anyway over time - once 'wires only' VDSL2 is finalised, I expect ISPs will increasingly move to providing multi-mode equipment which works equally well on the various flavours of ADSL and VDSL2, as well as having an Ethernet port to connect to an FTTP ONT and at least one voice port for voice over FTTC.
One unknown in all this is how fast FTTP roll-out will happen, and whether Ofcom will give regulatory approval for BT Openreach to discontinue metallic local loops and traditional voice service when all users have been migrated to FTTP (i.e. Fibre Only Exchanges). The obvious benefit of allowing this is that BT seem destined to have fewer FTTx aggregation points than there are existing exchanges at present, offering savings to BT by abandoning disused exchanges and to other operators from having fewer aggregation nodes to serve.
I expect universal FTTP to take several decades, even with the prospect of consumers helping BT with the costs of upgrading areas to FTTP via the FTTP On Demand product and the inevitable development over the next few years of FTTP or FTTB solutions for blocks of flats and apartments. The problem will be the diminishing proportion of connections that are disproportionately expensive to upgrade to fibre, though it will become standard to install fibre at the same time as - or instead of - replacing life-expired copper infrastructure. All this means there will be a case in the medium term for abandoning ADSL in favour of universal VDSL2, offering the highest speeds physically possible on copper twisted pairs. There may be some incremental advancements still to come in VDSL2, but I do not foresee another major generation of DSL. When there is demand for >100Mbits/second, that will become a driver for FTTP deployment - at the moment, there really isn't a killer application for such connections.