The problem with this 0day argument its fatalistic and there is no way to verify it. There is are always 0days in the kernel too but that doesn’t mean we should abandon all hope and stop developing?
Understand ricochet is also written in memory safe python (though migrating to Go) and written with security in mind. There are many other base Debian packages that are not. If there is something that’s a weaker link its probably something else.
Worst case scenario it won’t affect anyone who doesn’t run it and so I wouldn’t consider it affecting “all Whonix users”
Matrix is interesting but can you suggest a desktop client already packaged for Debian?