“Let's stay friends” is one of the most common phrases at the end of a relationship, and also one of the least examined. Sometimes it's a genuine, well-considered goal. More often, in the immediate aftermath of a breakup, it's a way to soften the loss — keeping a version of access to someone you're not ready to fully let go of, even if a clean break would actually serve you better.
A few honest questions can clarify whether staying friends is realistic. Has enough time passed that you can spend time together without one of you hoping it leads back to the relationship? Are you both actually over the romantic and physical dimension of the relationship, not just saying you are? Can you picture the other person dating someone new without it derailing you?
Friendship immediately after a breakup, before those questions have honest answers, tends to function as a slow-motion version of the relationship rather than a genuine new dynamic — which usually just prolongs the grieving process for one or both people, without either the clarity of a full breakup or the benefits of a real friendship.
A full period of no contact, lasting long enough for both people to genuinely rebuild independent lives, is often what actually makes a later friendship possible — not a rule against friendship itself, but against rushing into it before either person is really ready.