Assuming a long-term investment horizon, the reality is that at near bull market tops, you have lower expected future returns and lower yields from your portfolio. Yes, you have more money in the account, but the money is actually worthless.
Lots of investors, including many physicians I know, sold out at the market lows of 2008-2009. They looked at their recent losses and compared their balances to how much they used to have and how much they needed in retirement.
I’m not advocating that you somehow try to time the market. Remember, a bear market is a semi-regular occurrence. You will lose some money in bonds. You will lose some money in stocks. You will lose some money in real estate.
You will lose some money in precious metals. The only reason to change your asset allocation (investment plan) is in response to changes in your need, ability, and desire to take risk.