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.