Trillian was just a UI that put all your contacts in the same window. You couldn't talk across protocols, or merge the same user contact across multiple protocols.

Think of Matrix as a unified protocol from which AIM, MSN, ICQ would have all been based. And so if someone is on AIM but you registered on MSN, you can still talk. And at the fundamental level, it looks like IRC. It is the opportunity to re-baseline everything on a standard that is open and supports end to end encryption.

So while bridges would be needed today, the idea is that some time in the future these services would re-baseline on the Matrix protocol, or be displaced for whatever market reason by a startup that chose to baseline on Matrix.