Sure, WhatsApp can go rogue at any time. Signal app is obviously better. However, at least we have a possibility to find out if WhatsApp goes evil - decompilation, protocol analysis are still available. I wouldn't also discount fear of reputation losses if E2E vanishes from WhatsApp.

Compared to maybe-evil-in-the-future WhatsApp, default Telegram is already evil, because they have access to all communication right now. What do (or will) they do with all that data?

(Opt-in E2E - and only for 1-on-1 chats - is almost useless. Encryption should be the default.)