I just see it as something else to add problems. Last night for example there was some stepping up going on at King’s Cross, which worked as everything involved was 387s. This time next year as well as 387 and 700/0 there will now be a third type of train involved as well. The service is ropey enough as it is nowadays - yesterday was very rough round the edges pretty much all day for little obvious reason - so anything that makes things more complicated is a negative step.
I’m not saying it’s a massive problem, just that it’s introducing a new issue that hasn’t existed since 2018, to add to a very long list of performance issues that the GN now has.
In short, they struggle to make the timetable work now, so having an inflexibility added is going to be unhelpful, especially if the fleet size is still going to be cut to the bone to allow uplifts elsewhere, as seems to be the case.
Worth remembering the GN still won’t have as many units (43) as they did in 2018 with the 365s (50).