The answer to the first question: Probably retired, emerging for an infrequent role in a Coppola movie or a supporting turn on Broadway.
The answer to the second: Streepless and Pacinoless, at the very least.
John Cazale would've been 71 years old today. After a prolific theatre career, he made his first feature film in 1972 at the age of 37. He'd make four more through 1978, when he died of bone cancer. All of his movies were best picture nominees, two of them winners (The Godfathers). Pacino and Cazale were acting partners, and who knows where Al would be now if he hadn't gone with John, arm in arm, from Provincetown to New York in 1968. Nine years later, Cazale's lover Meryl Streep took her career-igniting role in The Deer Hunter just to be close to him as he died.
Cazale was the Great Enabler -- quiet, kind, not so much a chameleon as a phantom. Five classic films, and he had not a moment of volume in any of them. Instead, actors used him as a sounding board and entered the pantheon of greats. Cazale played hapless to Pacino's lethalness in The Godfathers. In Dog Day Afternoon, it was the same passive-and-active dichotomy for the pair (the way he delivers his sad-sack answer to the question "So what country do you wanna go to?" is representative of his onscreen humility). In The Conversation and The Deer Hunter, Cazale's roles (and performances) seem like an afterthought compared to Gene Hackman in the first and Christopher Walken in the second.
His roles were in the background. A lesser actor would've tried to throttle them forward. Because he didn't, we still remember him today.