Just watched The Swimmer and I didnt realize how much it inspired Under The Silver Lake which I really liked. It's also similar in character progression to films like Blue Jasmine or even Falling Down in a way. Are there other films of it's kind from before its time?
Another one that I often say would make a good double feature is Seconds (1966) - which is in my top 10 English language of the 60s and is imo an even better movie.
Like The Swimmer it shows the darker underside of success and happiness and also like it casts a strong American film male from the 50s (Rock Hudson here) who is drifting towards mid-life crisis. Both movies were crucial in anticipating what was coming in the 70s - in this sort of alienation and creeping malaise of middle age - and they tied into things happening in other mediums too - both are based on books and there's a lot of great stuff in this era of the curdling of the American Dream (The Twilight Zone - many episodes, A Fan's Notes (novel), Revolutionary Road (novel))