To Donal's point, it is a fairly safe bet that no further R&A events will go to Portmarnock if it remains a Men Only club: the contract for the 2019 Amateur had been signed long before equal membership access for men and women became a sine qua non of hosting R&A championships.
Lytham's days as a regular Open venue seem over, certainly for the time being: five Lytham Opens between 1973 and 2001 is, in retrospect, remarkable, given the space issues on the Lytham site, although I have heard members of the championship committee argue, robustly, that as a test of shot-making RLStA remains overall one of the two or three best challenges on the rota.
The other 'marginal' course in spectator-number terms is, arguably, Muirfield. But finding another Open venue south of Hoylake that isn't RStG remains perhaps the biggest imperative in terms of the demographic composition of the UK as a whole. And also much the toughest.
I think the Muirfield issue is all about access. The courses that get the really big attendances (apart from St Andrews) tend to have reasonable train service. Certainly that is true of the Lancashire courses, and Sandwich is best accessed by train for sure). Muirfield is a pig to get in and out of. When Ernie won at Muirfield in 2002, it took me four hours to get out of the car park and into Edinburgh (where I was staying with friends). It's four miles down some very little roads from Drem station to Muirfield.
Was at the 2013 Open there with Phil winning and was part of an SPG Moments event and they had several large vans for us, but took about 3 hours to get back to Edinburgh and we stayed all the way until after the trophy presentation. Saw Scott Van Pelt taking some selfies on 18 fairway and said hello, he was a cool guy. It was also hot that week. We played Turnberry Friday and it was hot.