The rationale given was the fact that the Diesel Euro models HSE and above all shared the same power plant (SDV6) and thus to run the line for the SE meant a significant change to the production line. As you point out other markets still have the TDV6, so not certain how true this is. The dealer did indicate that originally, and for the face life model of the Discovery 2014 model, the SDV6 was originally proposed, but was dropped. On going having 2 x diesel engine with the same cc just does not seem logical; so to answer your question I think the days are numbered for the TDV6 - but for how long, who can tell.
I would tend to believe that with RR production running at a maximum, to run off units with a list price, and in the case of the UK £9k less than what they could achieve, just does not make economic sense.
I wanted the SE simply as this was 'old technology' and history tells me that gremlins are usually ironed out and the engineering is tried and trusted Thus the engine, the Terrain Response, and 4x4 system etc are all old school. Coupled to the fact that I did not need the additional whistles and bells the £9k saving was to go to miss.