This is a misnomer. The phrase should really be, "Let's become friends."

It's a process. One that should be taken together, not alone. The person offering the phrase, though, wants the process to be an instantaneous change.

Well, I got news for you: It's not. You can't expect me to re-evaluate all my feelings, thoughts, memories at one instant assuming the stature of a friend.

Just because it would be convenient for you if I suddenly dropped off your dating list (so you can date others, maybe?) doesn't mean that it's going to happen. Sure, you've been thinking about it for a long time, maybe even weeks, and I even noticed some changes in you, but you have to allow me the same amount of time (if not longer).

Don't assume that since you've gone through the process that I don't need to.

And maybe next time you will discuss your feelings and intentions as you have them, when you have them, instead of waiting until you decided you wanted a change in our relationship.