I used decline 2 times. No I do not think returns will be average normal, A reasonable, normal return for bonds will cause many people to move from the stock market to bonds, and there will be more supply than demand in the stock market.That should cause a decline.
It is also possible that the stock market is overvalued now. Many people believe it is. We have had long periods of several years in the past where the stock market has lost value or gone sideways. Look at a graph of the last 100 years of the Dow.
Look at the Dow from 65 to 83. It finally got back to 65 levels in 90,95, that's 25+ years. I recall a lot of agony in the 70s.
29 to 58, 59 was similar. That is a lot of years, and we currently have been in a long bull market.
Dow Jones - DJIA - 100 Year Historical Chart
Still there are always things that can effect the market. War is often good for the market (see WW2) while a very large decline in the human population would be devastating. Not really material, but the US has a very low birth rate. We are depending on immigration to expand our population and economy. A pandemic with an extreme number of deaths would be catastrophic to the economy.
So for me diversification means stocks, bonds, cash, and real estate. OTOH, I've seen each of those tank except for cash. Things might change from "how much can I make?" To "how little can I lose?".
I have been fortunate in my life. I started investing in Stocks in the early 80s when I was 30, and the market has basically risen ever since. The. Biggest reason I' ve made money in stocks is DUMB Luck. Right place, right time. A degree in Finance might have helped a little.