I wouldn't be so sure. If you create a Matrix account and join an IRC channel, your username[m] will never leave the channel, even if you never use Matrix again. I made a few test accounts and it was rather difficult to figure out how leave IRC channels properly if you decide not to use Matrix anymore.

If you part the matrix room, you’ll part the IRC channel.

You can also disconnect from IRC entirely from the bridge admin room.

Finally, if someone joins a channel via Matrix and then disappears without parting or disconnecting, then after 30d we’ll boot them off IRC anyway.