In other words, what path her career would have taken in the 1960s and into the 1970s? She was still in demand, and even though the culture and industry were about to take big shifts she was only twenty-seven when she left the business in 1956.
Would Grace have remained an A-list star into the mid 1960s if she gravitated towards the right kinds of material and directors even beyond?
Well, she certainly quit while she was in great demand, and still a great beauty.
An obvious answer had Kelly kept working: she would have made more films for Alfred Hitchcock, and hopefully during the "good years" that he still had ahead of him, and not the years of decline in the late sixties.
Hitchcock actually got Princess Grace to give a public "Yes" to returning in movies for "Marnie" (1964), but she backed out for various reasons, and it was probably a good move: the character was a frigid, unlikeable, un-charming mess, and the movie that WAS made(with Tippi Hedren) was not that good.
Hitch also pitched the Eva Marie Saint role in North by Northwest to Princess Grace and THAT would have been great: a reunion with Cary Grant from To Catch a Thief in what would become the greatest Hitchcock chase thriller ever made (picture Grace Kelly up on Mount Rushmore with Cary.)
Hitchcock evidently would have "pitched" Grace Kelly all of his movies after To Catch a Thief and she would have fit many of them: The Trouble With Harry, The Wrong Man(in her dowdy Country Girl mode), Vertigo, North by Northwest(which would have been the best fit)...possibly The Birds, and if need be, maybe Marnie after all. I could see Kelly "holding her looks" long enough to work with Paul Newman in Torn Curtain...but that would be the last Hitchcock for her.
Grace Kelly would NOT have fit: The Man Who Knew Too Much(the lead required a singer, and Hitch got Doris Day) or Psycho(I don't see Grace Kelly in that real estate office, or that shower.)
Grace Kelly had children and gained some weight after her marriage, but there are 1972 photographs of her accompanying Hitchcock to his Frenzy premiere at Cannes (in her new home town) and she looked quite beautiful there (with long, LONG 1972 hair.)
Kelly probably felt correct in her choice as she watched actresses like Joan Crawford, Ingrid Bergman, and Ava Gardner play out middle aged versions of themselves in the 60's -- the health, food, and beauty regimens of the time didn't allow women to "age gracefully" on screen.
Still, I will assume that Grace Kelly could have kept going right on through the fifties(easily) and much of the sixties. Other than the Hitchcock movies, I can see her in Audrey Hepburn roles, and Eva Marie Saint roles(not va va voom roles like Marilyn Monroe's and Kim Novak's), and possibly playing more mothers. How about Mrs. Robinson in The Graduate?