In other words, you want your chats with news and memes E2E by default, so when you chat about something sensitive you a) don't have to do anything, b) won't forget about it until it's too late and c) won't give away the fact that this particular conversation suddenly went private.

Telegram management is weirdly stubborn af in this regard. Which could be either "we know better" syndrome, simple ignorance, or even malice. They, however, undeniably know their ways with UX and marketing, so, once again, as in an old Russian proverb, we end up with a barrel of honey, with a spoonful of tar - the nicest-looking but crappily implemented tech always wins.