Basically irregardless of the distro you will generally get the same access to the same tools. So, switching to a different base distro wont help, as Ubuntu has access to most software that is available. If it isn’t in the repos, you can simply install a deb, or add a ppa.
I also like Thunderbird, but understand that you may have the need for a more robust e-mail client.
Of course they are working on both a calendar and e-mail client currently. They are built for Unity 8, though and probably wont be available until 14.04 (though you can install unity8 in 13.10, and the ubuntu touch apps, as well)
They are not ignoring the desktop at all, or those needs. They are building scalable multi-device apps, and working on integrating unity8 with the desktop. So all work done for the phone will benefit the desktop. They use the same backends as the desktop uses. So any improvement to the backend (libraries, etc…) will most definitely benefit the desktop as well!
Hang in there and try some of the other mail clients, there may be some Java based, cross platform ones that may do what you need for now. Or of course Wine/or VM if you really need a certain program.