Addressing only one point, not your main one which I agree with:

> And also more likely, someone they talk to will forget to switch it on and just blab everything into cleartext anyway.

I expect that if you enable a Telegram Secret Chat with Bob, Bob cannot unilaterally un-secret it. I would be very surprised if that was the case.

Of course Bob can then share the contents with Carol via an un-encrypted channel. But every encrypted channel has that weakness.