Account Summary Screen shows List of accounts, in one view, with sum of Unread and starred messages. From here a single tap to the account allows access accordingly. If there are no changes to message count it will be visible without the need of a single tap (simply unlocking the screen to the K9 app).
People may have multiple accounts (I have over 10, others have more), and to swipe-tap drop down-select, on each account to just see if there is anything is very resource heavy (consider a resource being time, and finger pressing).
The option for UNIFIED mailbox is not an option given these mailboxes may all be different providers and serve different purposes; some may be personal mailboxes, some may be notification mailboxes that receive MANY emails that, at time of reading, may not me relevant at that time of day (and simply clog the view).
Further more, a single button tap to force refresh the accounts on the list is beneficial as well as the mailbox storage size (I use this feature a lot). And yes personally I view and remember the counts against the different accounts which helps determine where there are updates.
Also, a single tap to go to either the INBOX - all, unread or starred, or to the folder list of such account (by choice) from the summary screen is a major part of my use.
And now my beef:
To counter the question of “why should you bring it back”, I would like to ask “why did you take it away”? What purpose did it serve? What customer research/enquiry did you make that concluded it was surplus to requirements and the alternative (to make the app the same as all other stock mail apps) is better?
The Account List with One-Tap entry was the USP (Unique Selling Point) of this app that made it stand out and attractive. Now it is the same as all other apps (including Gmail which also allows multiple 3rd party accounts) and therefore no longer makes it standout and doesnt sell itself with any benefits over the rest. And given most phones already have GMail app (forced) loaded - there is no longer any need to have this 2nd app.
One other thing: I know you said people simply saying “bring it back” is not sufficient to convince you to bring it back (and you actually want reasoning). Well, if a thousand people want to ‘bring it back’ they must have their reasons and the chorus of voices really should be enough to tell you that they have their reasons and having them all say why (the same reason) doesn’t validate or invalidate why the request should/shouldn’t be made - they wouldnt just ask for it if it was not important. Clearly by quantity of disgruntled users that has made their opinion known, it can be implied that SOMETHING is no longer sufficient for them. It’s true that the app is offered for free and people can take or leave whatever has been offered (no refunds apply) BUT there must be a reason you offer it and I can only conclude you do so for the users’ benefit and the pride of overseeing such work. So, surely, shouldnt the users comments/displeasure be heard (in threat of them leaving)?
For now I remain at 5.6 but the faff of ignoring automatic updates will eventually get the better and it is only a matter of time (sooner, I suspect) before I do eventually go to the Darkside of using Gmail or stock mail app.