I've seen my father manually clear all his whatsapp chats at the end of the week. 1 million is definitely enough for everyone but somebody who's not using telegram for chat.
As far as I understand it you can't scroll back further than a million, so if you were using this for notifications, you wouldn't be able to see what your n-1,000,000th notification was. I could be misinterpreting, but if I'm right I still don't see it as much of a problem.