That's just excuse making. The stations that these trains are going to be using, and the layout of the stations was (obviously) known to all involved long before the trains were specified. I know it upsets some people here if any criticism at all is ever levied on the rail industry, but to order bespoke lines for a service that don't fit the stations they're going to serve is astonishing incompetence. Did nobody at any point in the process say "Hang on, if a passenger gets off at the back door on Friday evening, then on Monday morning finds that the back door of the shiny new train doesn't open at their station any more, we're all going to look (even more) like complete idiots"? Yes, there are some Underground platforms that are a bit shorter than others, but the fact is that it's currently possible to have a set of trains on which all doors open at every station. How can going from that position to a position where that's no longer the case be seen as anything other than a (completely avoidable) retrograde step?
This sounds a little unfair. I can't speak for the decision makers here (and I'm not sure it matters whether they are DFT or one of th Train Operating Companies) but I would imagine it would go something like this.
1. We are replaceing the trains on the line. Ideally, we would like longer trains so we can take more people.
2. Unfortunately the platforms at a few stations are too short for the length of train we would ideally like to use.
3. Where we can, and it is economial to do so, let's lengthen the platforms.
4. It seems that there are still some platforms where it is not economically or physically feasible to lengthen the platform at a few of the stations.
5. Therefore, we have the following choices: a) abandon lengthening the trains, because there are a smal number of platforms were it cannot be made to work; b) continue with the train lengthening project (because it is sorely needed) but abandon stopping the trains at the stations where platform lengthening is not possible; c) continue with the train lengthening project (because it is sorely needed) but put in place SDO which will result in marginal inconviniece for a small number of people who will have to walk a bit further to get off the train.
Faced with that way of thinking, I suspect that most people who choose option C, because it results in the greatest gain for the most people, with negligable inconvinience to a small number of people.