If ECI are cheaper why aren't BT using them now? Even better why were BDUK not using them if the public was funding them!
Benefit of hindsight? BDUK was rolled out several years after the main FTTC rollout. I may be wrong but afaik its only the BDUK cabs that perform vectoring.
Since original rollout Huawei has developed a system based vectoring module that could be slotted into the larger Huwaei cabs.
What was one of ECI cabs major selling points (small footprint) became a major disadvantage because there was no room on the M41's back plane to insert a separate module for vectoring.
As to G.INP I've still no idea what the problem is because Openreach aren't publicly saying but all the rumours seem to say its something to do with modem incompatibility.
Being on an ECI cab myself I'm not too chuffed about no G.INP (which worked fine here during the brief period it was enabled). My line would happily work at 3dB, but that ain't happening. No prospect of vectoring which could give me back the >40Mbps overheads I've lost due to cross-talk... and now the unlikelihood of SRA... and on a line just to long to ever be able to benefit from g.fast. There's no VM here.
Yeah it's a bummer, but at the end of the day I try console myself with the fact that FTTC has been here now for ~6yrs and once upon a time I was able to get the full 80/20 with a faster connection than most at a time when the vast majority of people were still on adsl.