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 realistically happen if the world entered a very prolonged period of stagnation/decline? I’ve considered some impacts (for example pension funds would be heavily impacted and countries in high debt would be in substantially more debt), but I’d like people that are more experienced in economics to let me know their thoughts. Finally, in terms of market growth, would inflation alone guarantee a small but relatively certain increase? In that case would stagnation essentially be the economy shrinking? Thanks all