If you want to keep the branding of Signal being the secure app, you need to make sure that all Signal users are actually using a secure version of Signal.
If an insecure fork (like this one) becomes too popular, most groups will have at least one member using it, and then the security is gone.
Sure, this is HN, we know one of the effects of locking the ecosystem and coloring in-system messages differently is to encourage people to be in the ecosystem.
At the same time, you ALSO need to consider that obviously there will be leaks.
Malicious/advertising apps will target the new messaging interface to gain more data on their victims, etc.
Locking down a platform is not an acceptable solution to the above conundrum - it doesn't matter if the user is using an official device/app whatever if they are untrusted. They can always turn around and leak everything you say without any technical measures.
Should we have no security? No, if you want to color messages differently based on perceived platform, fine. This is just an illustration that no technical measures can replace the fundamental trust necessary in these types of situations.
The third-party federation problem is real, but the vulnerability caused by TeleMessage isn't solved by removing federation.
I believe the main criticism against Signal is that they should focus on getting widespread traction of secure messaging, and that perhaps the brand can be a relatively distant concern.