Eh, I don't know if it was illegitimate or not considering he allowed abuse to carry on on the platform for years and due to no encryption it was well known about.
I hope either people using the platform or telegram itself learn their lesson and the people using it leave for better platforms or telegram finally add meaningful encryption and stop pretending that they can sit back and do nothing.
No, I'm not suggesting abuse should be allowed to carry on just because it moved to encrypted messengers, nor that all the 'illegal' things that happened on it are bad (something being illegal doesn't mean it is unethical inherently), just that those doing nothing unethical deserve protection and telegram doesn't provide it.
As for the rest well, they deserve to be taken down and kept well away from anything that would enable them to abuse.
SimpleX isn't a fun or customisable messenger, so most people will not use it until it has the other features that Telegram or Signal have. Its funding sources/business structure are suspect, it could easily end up sold off and/or enshitified.
Also, SimpleX doesn't have easy or in some cases possible (as it isn't possible to access the file structure completely on some OSs) profile/message syncing on all platforms.
Finally, SimpleX isn't fully compatible with all devices and sticks them in an awful looking mode with no prioritised plans to fix this, Telegram and a few others don't have this problem.
Briar isn't on all platforms yet, so that's why there.
You're not there for fun lol, you're there to pirate stuff. Sure, using Signal to pirate would be fine too but your anonymity depends on how long they honor their word of "no logs". Use a desktop version/ run a VM if you're on MacOS or something.
Everybody hates me when I say it but Apple users should really reconsider their choices if they are at all interested in privacy. Go get a cheap Android device, hope it has a kernel beyond 5.10 and KernelSU it.