Thunderbird has been more than "an email client" for so long that people made the exact same kind of complaint when RSS support was added.

Thunderbird is a messaging/communications client, and way more people will benefit from Matrix support than would ever benefit from Maildir support.

Further, my understanding from the Thunderbird team's comms is that they are very much in control of their own destiny. I don't get the sense that Mozilla has much say in Thunderbird's roadmap (or even cares) at this point.