From the research I have been doing over the past year or two, the UK stock market has shown about 5% real return (after inflation) over the long term. I think the USA stock market is slightly better.

I am a total novice, and have fairly little invested, but those are the sort of numbers that pension projects are based on.

If you can keep you personal burn rate low, and over time invest a significant amount of money at 5% return for 10 years, the interest really starts to snowball.

"by much" was a key in my comment.

If the average return from an index fund is 7% and inflation is, say, 3%, then you're making an extra 4%.

If you're investing $100k, you'll double the value of your money in 20 years. That's not great for someone who doesn't have much other cash and could use that $100k over those 20 years for significant lifestyle improvements.