In general the point I am attempting here is:
End to end encryption is more secure when used offline in a safe environment.
An end to end encryption system requires a critical secret to be effective. This secret has to be available to the user but not to anyone else. Loss of the secret usually means exposure of the encrypted messages.
Something like an instant messaging system has to be, well, instant. As a result you can't protect the critical secret very well. You can't expect a user to enter a passphrase every time a message comes in. You can't expect them to attach and authorize a hardware device either. You can't expect them to do something so cumbersome even for a series of messages. That would destroy the value of instant messaging.
Typically a instant messaging system installed on a computer system provides little or no protection to the critical secret. The secret is exposed any time the computer is running. Typically anyone at the computer can look through old messages and impersonate the user just by sitting down and opening the instant messaging program. A more advanced attacker can get the critical secret out of memory or off the disk. Locking down the system has the side effect of reducing the value of the instant messaging.
Instant messaging is more valuable on mobile devices. Here things are worse. A smart phone is on all the time and the critical secret is exposed all the time. The protection of the critical secret is in effect the protection provided by the phone itself. Something as simple as distracting the user and walking off with their unlocked phone makes for a practical attack.
By committing to only doing your messaging from time to time in a safe environment you significantly increase the security of that messaging. With something like encrypted email the critical secret can be protected with a strong passphrase. Such measures do not destroy the value of encrypted email as they do with instant messaging.
Even further measures can be taken for greater security. A dedicated hardware device can be used for your encrypted email. This device can take the form of a USB key. It could even be a complete computer system kept in an area under your control.
The point here is that the lesser security of instant messaging is inherent. It is an unavoidable side effect of the medium.