Groups.io yields the funtionality of a mailing list with the mails arriving in 2 seconds.
Logging in at groups.io also delivers a forum functionality with modern aspects such as chats and wiki. Please have a look. Just type in any password (and keep it in mind). You'll get an email for first validation.
Groups.io supports for hashtags. With young people instagram is currently the way to go, so for the future hashtagging might be important and my guess is that hashtags won't disappear even if instagram would disappear.
Groups.io supports for a user directory: you may set whether your profile can be seen from other members of the group.
Groups.io would also allows to do an intergration with other services.
With respect to group management: everybody can get owner or moderator.
All together I'd say that groups.io seems to be a modern alternative to yahoo.
There is however also a backside: The data limitation of 1GB for files, pictures AND message attachements.
This would mean to stay on yahoo at least for the files or to find another solution for them.
I am completely open how to proceed:
+ Just to stay with yahoo and beeing happy that responsetime is now 5 minutes and (hopefully) messages do not anylonger disappear. Therefor maybe some new moderators are needed. For Middle European Timezone I would be willing to serve if there are no others.
+ going a hybrid approach with groups.io as messaging and yahoo as file server which seems to work there still very smoothly including email within seconds.
+ going another hybrid approach with groups.io as messaging and some page like sliderule.org as file server
