If just using encryption is good enough for you, then why not use Facebook Messanger, Google Hangouts or whatever they call it these days, or WeChat? You can verify their use of encryption with Wireshark.

You argue that Telegram is great because it's open source, but what difference does that really make when it's a centralized service and that centralized service has access to all the plaintext? So what if the client is open source? What does that actually help us verify in this case?