I understand what RCS is and I don't understand why it matters.

Everything about the concept of a phone number is confusing to me. It's a string of digits that if someone guesses, they can activate the most active notification your phone has (ringing), at any time, no matter if you know them or not. Better yet, depending on your notification and MMS app settings, they might be able to make a dick pic appear on your lock screen on a whim - big spammers of this seem to get marked by the carriers and apps pretty quickly, but for a more targeted one off, still easy.

As opposed to tcp/IP based chat apps that basically require a bilateral human-initiated handshake before someone can message you...