A single instance of Laconica probably won't support millions of users, but the whole point IMO is to federate smaller Laconica sites rather than building a huge centralized monopoly.
Those who don't understand are doomed to re-invent. Poorly.
I'm speaking specifically of "PubSub", where you can publish your updates and they get sent out only to subscribers.
Note that twitter does not otherwise match a conventional IM message, nor does it match a conventional conference room. It does precisely match the PubSub use case, and I still hope to see something based on that. Laconica isn't wrong to ignore it, per se, but they will end up having to do (or have already done) a lot of very, very redundant specification as a result of ignoring it. (I'd think they'd still have been better off taking an existing XMPP server, fixing the pubsub to do what they want, and building from there.)