In an index fund, the stocks contained in the index will on average pay out a certain dividend and turn a certain profit that isn't squandered on worthless projects.
This last part is what makes stocks increase in market value long-term, and which also makes it reasonable to sell a small proportion of your shares every year as a passive income.
If the dividends and your sales constitute less than 3-4% of your funds' total market value each year, you can expect your portfolio not to decrease in value long-term.