Until the extra housing is built around Wantage/Grove, I would suggest that there is no untapped pax wanting to travel, as they all currently travel via Didcot - see the car park usage there!
So building that station will wait until a) the houses are being built, and b) capacity between Swindon/Didcot is capable of carrying stopping trains. The 387s to Didcot extended through to Swindon would be the obvious carrier towards London, with EWR being the cross-country carrier you are discussing above.
Wantage and Grove had a combined population of 18,000 in 2011 how big do you think a place needs to get to support a station? Melksham had 14,000 in 2011. Didcot had a population of 25,000 in 2011, but kept a station when it was far smaller than Wantage and Grove are now.
Surely it's not just about the gross population living next to a station though - it's about how many of those people use the train on a regular basis, and the 'hinterland' that drive/cycle/take the bus to the station. Plenty of people in places like Devizes and Marlborough who drive a fair distance to a station on a fairly regular basis. There must be plenty of relatively small towns and villages in the South East of England where a significant proportion of the town uses the train to get to work/school/shop and thus the station is economically viable.
Also, we shouldn't forget that better services will attract new residents and businesses, and also that even if rail users just drive to a town's station to park, in the long-run they will tend to help the local economy as they will be more inclined use local shops/restaurants/cafes/childcare/garages etc.
