What are the downsides to telegram providing default E2EE? Seems like a no brainer to have it as a default feature for the product.

I think they don't support cross-device syncing or automatic backups of E2EE chats, so it's about minimising friction by default. Telegrams main focus is UX, unlike Signal which prioritizes security at the expense of UX.

There's nothing in Telegram that couldn't be implemented with security in mind. They just lack the expertise in designing cryptographic protocols that offer those features, and Durov is too proud to consult experts in helping improve the design. Well, now he gets to enjoy French hospitality.

Their focus is on UX more than security. The app is super snappy and supports group chats with hundreds of thousands of participants.