Isolation in Urban Life

In the following excerpt from a fictional novel, the protagonist confronts the bleak realities of urban life. The text explores themes of isolation and the struggle for belonging. Read the passage carefully:

“As the rain drizzled down, painting the streets a melancholy gray, Sarah felt more invisible than ever. Crowds of hurried commuters brushed past her, their faces a blur of indifference. In this bustling city, where the noise of ambition collided with the silence of desolation, she often wondered if anyone would notice if she simply vanished. It was in these moments of solitude surrounded by throngs of people that the weight of her isolation pressed upon her like an unwanted shroud, a constant reminder of her place in a world that seemed eager to forget her.”

What is the central idea conveyed in this passage?