unity-mail application gone?

Well, Unity 8 will not be part of ubuntu 16.04 so it would be nice to have unity-mail back on.
The second reason in your list above is like comparing apples with oranges. Thunderbird is a full e-mail client and, to be honest, it stinks as a notifier. Not to mention it's resource heavy (I gave up using it > 8 years ago)
Nobody can have arguments against your first reason unfortunately :)
In case you find some 25 hour days in your back pocket, please consider keeping unity-mail alive (at least an initial xenial version without further maintenance - if possible)

In the mean time we can use only two alternatives that come close and I know of:
1. mailnag - lacks a visible counter of unread emails unless you use gnome, and
2. geary - half way between client and notifier - which has a counter but works with new mail not unread mail (doesn't work with toggling read/unread, restarting geary/system etc).
Unity-mail is much better for what it is made for.

Thanks!

For Cim Man (and others):

https://tari.in/www/software/unitymail/

Sorry for the typo in your name...

Thanks Dmitry Shachnev, that solved my question.

Thank you!
I'm at home again :)