Could SPY actually lose 40% in real terms like it did in the 70s?

Just read something about how the S&P 500 lost like 40% in real terms between 1968 and 1982 and now people are saying it could happen again. I had to read that twice because the index was technically up some of those years but inflation just ate everything quietly. I guess my question is how do you even protect against that. Like if SPY is at all-time highs on paper but inflation is running hot, are you actually making money or just treading water? I never really thought about returns in real terms before, I always just looked at the number going up or down. Also 14 years is a really long time to basically go nowhere after inflation. That is longer than I have been paying attention to markets at all. Does anyone here actually factor inflation into how they think about index performance or do you mostly just watch the nominal price?

Not financial advice.