The full implementation of Real ID has been delayed due to COVID-19, eventually all US citizens will need to have either Real ID or a US Passport to travel. But is Real ID a substitute for a passport? Do you need both? In short, a resounding NO!
While Real ID will help you in domestic travel situations and in some situations abroad such as cruises and all-inclusive resorts, it will not help you if you need to diverge from your original plans in any way. For example, if you are on a cruise, and suddenly need to return back home, you will not be able to board a plane in a foreign country with a Real ID. You will only be able to divert your trip with a valid passport. Also, in most cases, you want to ensure that your passport has more than six (6) months before it expires. Many travel vendors will not allow you to board with less time before expiration, again as a way to prevent problems from unforeseen circumstances.
My final reason is a financial issue. If you are going to pay for a passport for foreign travel, why should you pay the additional money for Real ID. A passport will serve the same benefits as Real ID, but it does not work the other direction.