That's not a given. Visionary people sometimes go mildly insane over time. Source: worked at VKontakte and Telegram and watched it slowly happen to Durov.
Of course, I have nothing against being able to delete your own messages for both. A time limit within which you can do that is nice but I can live without one. But Telegram lets you delete every message in any PM conversation, including incoming ones, for both people. Or even the entire conversation, and make it disappear from the other person's list without a trace.