Almost all popular secure messenger systems are run by a single
legal entity.
e.g. WhatsApp by Meta, Signal by Whisper Systems, Telegram etc

Well, these generally are useful for solving the message delivery
problem - locating the device that your contact is using in order to
ensure they receive the message.

To do this they create a client and implement end to end encryption.

Governments would like to force them to implement client side scanning
of messages before they are encrypted and sent.
Because they are legal entities then they are bound by law to obey the
Government in the legal jurisdiction where their clients are used.

Maybe a solution to this is to create a client that isn't owned or
managed by any of these legal entities - just an open source app
published on the internet that anyone can download.

The client would implement end to end encryption but then be able to
use ANY of the messengers simply as a transport for it's encrypted
message.
It could even use SMS as the message would be encrypted BEFORE being
sent using SMS.

No legal entity controlling the client = no Government ability to
enforce backdoors or client side scanning.