Don't think this has anything at all to do with ECI specifically. Openreach are probably hanging back a little bit pending availability of 512 line solutions.
Openreach haven't deployed new ECI kit in a while now. Seems that whenever ECI DSLAMs are full they just build a Huawei next to them. I've seen extended Huawei 96 and 288 cabinets, presumably to 128 and 384, so those guys are I think ripe for further upgrade as investment is ongoing but ECIs just get a new, Huawei, DSLAM.
With the Huawei upgrades there's actually absolutely no reason why Openreach wouldn't install upgraded Huawei and decommission the ECI. The availability of single DSLAM cabinet solutions that reach 512 lines makes this an option. Install a new Huawei to replace everything, migrate everyone over, write off the old cabinet, no more support contract or operational costs on it and an accounting gain.
Getting Huawei kit to 512 lines means another 2 line card slots, presumably via a remote backplane connected to the existing kit via fibre and a power connector. They do that to the Huawei 96s as well those guys end up moving from originally 96 to 256 lines overall. Not too shabby.
This said, the cabinets will end up looking like Frankenstein's monster. The small Huaweis look weird when extended already.
Be interesting to see what they decide to do. They have a few options once Huawei have delivered the 512 line kit and, of course, Alcatel / Nokia make modular DSLAMs / ISAMs that'll do 192 lines per 4 slot shelf and allow multiple shelves per cabinet depending on needs so they could bring them into the VDSL 2 mix alongside making vectoring and G.fast from the same device an option.
Thanks to ejs for pointing out that briefing. This makes it clearer what Openreach are up to.