What happens if growth stops?

When watching personal finance videos I’ve seen commentators claim that if global/US index funds don’t continue growing over the long term then you’ll have a lot more to worry about than investments not growing - aka it would be an event that would cause multiple crises across the world and you’d be worrying about buying fresh water instead of the S&P 500. They use it as a way to reassure long term investors of the security of the market/index funds as a whole. My question is, what would realist