no one can guarantee a return of 15% in stocks, the historical return of the stock market is around 10% and includes the rapid growth stage of the usa economy through the mid to late 20th century, we now have a mature economy and there is no way to even expect that kind of return
Nobody can gurantee any rate of return but 10% is a good rule of thumb for stocks. The average annual gain for the S&P 500 for all years, 1977 thru 2000 (Carter thru Clinton) was 11.86%, based on closing prices on the last trading day of each year. Junior's average is about 4% so far. But your longer term worst case assumption of 4% is low. You could do better than that if you were to just put everything into bonds.
It might be true that US stock returns will continue to be lower, due to lingering damage brought about by the current administration, the state of the economy's maturity as you mentioned, and/or other dynamics. But opportunities are not limited to US equities, and investments elsewhere are more promising. This is particularly true when you consider non-US stocks are not based on the US dollar, which will continue to fall against other currencies. More specifically, I wish you would consider making international stock mutual funds part of your portfolio if they are not already.
It seems odd that your brokerage firm seems to have neglected mentioning this, and I wonder if they are trying to manage your expectations so that you won't contemplate better results than they actually deliver. You might consider talking to another firm to see if their advice differs.