As others have pointed out, yes it was seriously considered that Maria would die.
But it was ultimately decided (I'm not entirely sure if it came from a specific person who pushed the idea, or whether it was just unanimously decided, though I'm leaning towards the latter) that Maria would live because she's too strong to die. They figured the tragedy remained, as she would have to go on living without Tony.
"If there was a Mount Rushmore for Broadway scores, "West Side Story" would be front and center. It snaps, it crackles it pops! It surges with a roar, its energy and sheer life undiminished by the years" - NYPost reviewer Elisabeth Vincentelli