I think this is a really great discussion and I love it when you can see how we are collectively figuring out what the different needs of different types of users are and how to meet them.
As I see things at this point (and I have no stakes in any of the discussed features), the big question is whether it is better to make summary emails extremely customizable for the individual user so that it can be used in a very similar way as the daily digest, or to bring back/introduce digest emails in addition to summary emails.
On the surface, it seems like option 1 is the more elegant one, apparently keeping things simple and avoiding confusion between two similar looking email functions.
But I think that if the summary emails get a lot of additional options, this will probably be more confusing for users that having an additional email mode that is hidden somewhere further down in the user preferences. And, as has been pointed out, since the summary emails and the daily digest have quite distinct purposes, it’s probably better to keep them apart.
Also, thinking of the feature requests (plans?) for a [category based mailing list mode]
(Apply "mailing list mode" per category), it probably makes sense to see daily digests as a variation of mailinglist mode.