In 1998, a reasonable expectation was 50% per year, though you'd usually beat that if you invested in anything starting with a lower-case 'e'. Naturally, it all went away if you left it in, but that's beside the point.
Today, it's still not unrealistic to expect that money you put in today will make on average 5-10% per year over the next 40 years. If you're investing your money for a shorter period than that, it's not really retirement savings but speculation, which can be fun but is its own thing.
Regardless, I don't think I'd discourage people from saving in their 20s because you don't think the market is going to continue doing its thing. It'll be back, and the few hundred K that you can set aside in your 20s will do some amazing things over the course of your life if you invest it in index funds rather than granite countertops.
Eventually it's making more on its own than you're putting in.
This actually takes a long time.