As regards to your EWR point, that is certainly something I have heard of, but I understood that there were not sufficient paths on the GWML for that currently.
The rebuilt DNS offers connections at Newbury (a significant flow between Newbury/Didcot/Oxford on the A34) west on to the Berks and Hants, providing a much faster route to Somerset, Devon and Cornwall.
It also offers connections at Whitchurch on to the West of England line, providing Salisbury and the rest of the A303 corridor with a reasonable rail link to the business clusters along the A34 and M4 corridor, many not accessible by public transport. It could reconnect villages like Sutton Scotney and East Ilsley to the network as well and act as a railhead for small towns like Kingsclere - there has been a lot of housebuilding in the area and a lot of these areas will exponentially grow over the new few years.
Plus the freight that could be taken off the SWML/GWML/Reading to Basingstoke towards Bham/Cardiff/Bristol/Glasgow/Manchester is a big potential improvement, if a junction in 4 directions was built at Whitchurch to allow container traffic to head north/west via Salisbury, Westbury and Bristol over lesser used lines.
Basingstoke would be accessible via an interchange at Whitchurch, it is currently 14 mins between the 2. Assuming an average of a mile a min between Didcot and Whitchurch, it's roughly 25 miles, plus the 14 mins to Bstoke would meet the current 55mins between Oxford and Basingstoke easily.
I agree it is something to look in the long term, maybe 20 to 30 years, but A34 congestion is only going to get worse, road improvements are not electorally palatable and the growth has to be accommodated somehow.