Word of caution. Most of these apps are resource hogs on your Mac. I ended up using dedicated web browsers for the key ones and open sandboxed windows side by side in a separate Space for quick access to them.
Because email sucks? IMO, it became too popular, with users as well as spammers, and most people got tired of wasting their time searching for anything of value in their Inbox. Today virtually everyone I know uses instant messaging. I rarely get more than 1 or 2 personal emails a month.
Not really. All these chat apps were designed to be walled gardens.
There is hope for things to improve IF Apple is willing to adopt RCS, a secure replacement for SMS with E2EE. My bank, like most financial institutions, uses SMS messages for 2FA. This would be a major security improvement. At the very least, Apple should let us choose something besides Messages as our default chat client.
The giants, Apple, Facebook, etc. don’t work and play together. But here is some of what is available today. I used a few like Adium and Pidgin many years ago. Good luck with your search.