Openreach aren't. I'd say this is entirely in the hands of ECI.
I wonder how many complaints this has actually produced from Joe Public rather than the population of forums like these. Given it hasn't hit mainstream news I'm speculating not all that many in the grand scheme.
joe public because the CPs and openreach keep them in the dark 99% wont even know what g.inp is.
Instead they just notice a speed drop and moan at the isp (which even then is probably only a few %, maybe 5% max).
This would have been the same as the bodged hauwei rollout. But that few % was enough for them to come up with a fix.
There is some obvious cheap fix options available to openreach, however I think they wont take them up and instead put the onus on ECI, if ECI fail to deliver then I expect no g.inp.
As I said before, openreach testing of new configuration is clearly flawed, as well as the CPs policy of keeping end users in the dark, I know why they do this, they do this to manage expectations and to hide the truth from end users hence your comment about joe public not complaining because they got no clue whats going on, this is very wrong but it will continue as long as they get away with it.
ECI could quite easily come back to BT with something like this "we only support using compliant modem's with our product and you are responsible for enforcing this on your network".