I think Libera is (and only intends to be) restricted to catering to the pre-existing Freenode community; their sights are not particularly set on expanding/competing with newer platforms any more than the Freenode community's were. They just cater to people who like & want to stick to IRC, for whatever variety of reasons. I don't see any "growth" (nor "shrinkage") happening here beyond migration of users from Freenode.

Discord is not open-source, so I don't see how it's relevant; it's "communities" are primarily gamers & streamers. While there are some open-source communities based on Discord, the same goes for other closed-source platforms such as Slack & Telegram, and I'm sure the number of software development communities on Slack is (sadly) much larger than on Discord.

Of actual open-source options: Gitter, Discourse, Rocket, Mattermost, etc. - Matrix would seem to be competing relatively well.