Mk1 is highly unlikely to happen.
There are far too many modems out there (such as the ECI modem/HH5A etc) that cant do upstream G.INP.
When it was switched on, people with those modems saw a loss of ~10Mbps and increased latency.
The compromise with Mk2 was to make G.INP available to those lines which needed it and which were capable.
The 2016 rollout is different in that its causing PPP issues for modems which arent in some way compatible with the ECI rollout. This is service affecting because they either cannot get connected or they have problems remaining connected.
In some way it looks like Openreach may have anticipated there being a PPP issue and its why they issued this warning in Jan 2015 -
ECI modem - Issue 1. What they didn't seem to anticipate is that not only would it affect ECI modems with old f/w... but also other modems such as the Drayteks, or Fritzboxes.
Draytek seem to have admitted defeat knowing that their equipment cant be updated - hence offering circa £100 off a newer model. These didn't work on the Huawei cabs, but for some reason there seems to be a heck of a lot more people with Drayteks on ECI's. I think I only saw one or two posts about people having to consign drayteks to the bin last year.. but I seem to have seen half a dozen or so just in the past few days on the ECI's.
From what I can gather from one ISP, faults have taken a steep rise where they've had to send out OR engineers. I guess we are now seeing why Openreach are being more firm about Modem Conformance Testing.