Email is not only 1:(small N). Maillists do exist and and are used to facilitate discussions between a large amount of people via email. They are also often public so anonymous readers and search indexers can use them.

/all is certainly an interesting thing - default Active sorting calculates a rank based on the score and time of the latest comment, with decay over time. If Threads are connected they would dominate /all. But there can certainly be adjustments, we can create a new sorting style, and make it default. For example:

• Posts are deprioritized based on MAU or some similar metric. The larger the MAU, the lower the post is ranked assuming the same engagement. If the post got 100 upvotes on an instance with 1000 users, it's probably a much more interesting post, than the post that got 100 upvotes on an instance with 100 000 000 users.
• Posts are (de)prioritized based on instance source. For example setting Threads to -1000 would effectively remove it, setting Threads to -50 would allow you to see only super active posts. On the other hand if we want to see more content from less populated instance we might set it (i.e. german lemmy feddit.de) to the score of 100.
• Instances can provide a limited number or percentage of /all - i.e. after we got 10 posts from Threads, stop getting posts from this instance.