starting in the 1960s, and lasting well into the 1980s, there was a time honored tradition of casting, shall we say "fading", actresses from Hollywood's "golden age" on soap operas as a series of matriarchs and dowagers.
the characters were generally not really "front burner" in terms of actually driving plot. they were more a decorous "presence" that lent the often low budget productions cache. the pay scale was probably peanuts compared to what they had been earning at their peak in film. but dented egos could be salved by way of special "above the title" billing and lavish wardrobe. in many ways it was a "win win". the program was added prestige. and for the actress it was regular work at a time when film roles had largely dried up and there was not the constant revenue stream of "residuals" there is today with cable, streaming, dvd sales and so forth.
i'm not sure of Joan was officially the "first" in this particular brand of casting. but she was certainly a pioneer and one of the most distinctive.