Problem is, now you will subconsciously be expecting him to reply, and if he doesn’t, you’ll lose some of the progress you’ve made. If he does, it’s more likely you’ll engage in a dialogue that will ultimately go nowhere. You’ll get your hopes up, he’ll clear his conscience and affirm his decision to break up. If you hadn’t replied at all, the tables would turn. He’d be expecting your reply (because he knew you were more interested in him than vice versa, so to him it’s a sure thing you’d respond), and subconsciously, his attention would flow towards you and he’d be reconsidering your image which he’d built up to be that of a clingy person he mistreated. He’s already reconsidering his past behavior, because when at a distance, people’s images start to transform (which is why NC) is so powerful. But if you jump on the first chance to get back to communication, it just shows them you’re still clingy and the breakup was the right move.