So, Spritely's approach to vaporware avoidance is to break things into subprojects and show how we're making progress with demos. If some of the subprojects fail, but the others still exist and are useful independently, that's a big win.
Spritely Goblins is already quite usable, and does quite some interesting things: https://docs.racket-lang.org/goblins/index.html
Even if that's all that succeeds from Spritely (and I don't think it will), I think that's a useful contribution.
But seeing is believing, and skepticism is well warranted. All I can say is, I hope that in two years we can come back to this post and re-evaluate it and see how far Spritely is along. I hope we can prove ourselves. I'm working very hard to do so.
I don't actively use anything on the fediverse and still avoid Twitter, but I would still like to follow the project.
(Also, I hope we can get to the point where we dogfood Spritely for Spritely communications, but I think that's ~1-1.5 years out from now.)
I wouldn't necessarily be worried about having the mailing list be open for discussion, irc/gitlab/fediverse covers that pretty well.
That being the case, all the mailing list would need to do would be send the content/links of/to the fediverse posts whenever there's a large enough update to be newsworthy. I just brought it up because that's a way to make sure people can get notifications without having to sign up to new services.