Turnberry has no chance. The R&A want a minimum of 250k punters in Open week. There is no infrastructure around Turnberry and the course couldn't handle those numbers.
Muirfield and Royal Lytham are on the backburner. RL has spent a fortune altering holes, installing a new driving range, and freeing up space for stands and concessions. Next years Ladies Open may be a trial run.
Portmarnock is in Ireland. A separate nation state, and has its own perfectly good open championship. Taking The Open outside the UK would be the end of its status in many fans eyes. You wouldn't host the US Open in Mexico.
Portrush is guaranteed two more in quick succession as the Northern Ireland Government have subsidised the alterations and improved logistics (which are still poor).
Carnoustie is not "sexy" enough for the R&A marketing gurus, despite offering a magnificent test.
RSG is the safest venue. Nothing else in the area and London less than 60 miles away. Also brings in European fans.