> effectively using Matrix as a big decentralised IRC bouncer in the sky. No. In theory it is, but in practice it's a big centralized IRC bouncer in the sky where everyone is reliant on riot.im servers.
Not really: anyone can run an IRC bridge, and many people do. The #mozilla_#wherever:matrix.org namespace is the preexisting moznet bridge run by Matrix.org (not Riot.im), but anyone with their own server could also run their own bridge, much like they could run their own bouncer today. There is then additional decentralisation from the fact that the room history is replicated throughout all the participating Matrix…
Matrix is toxic to IRC.
Any widely used IRC channel that has matrix joined to it would do well to ban matrix if they want the channel to pratically continue to exist as a IRC channel.