I wish my N900 lasts forever, but it's wise to do some research about the competitors.

The most important thing for me is a comfortable hardware keyboard. I really lose my cool when I have to cope with all those on-screen keyboards.
I can give up much but I won't give up the hwkb.

Is there anything reasonable with a good hwkb out there in the market? (other than our N9x0s)

Only if you build one yourself. http://talk.maemo.org/showthread.php?t=95650

I use a BlackBerry passport, 3 row keyboard with the rest as on screen. I quite like it

There is also Blackberry Priv: http://www.gsmarena.com/blackberry_priv-7587.php

Samsung actually made this keyboard attachment for the Note 5. Don't know how well it worked but apparently the screen adjusted size when you attached it. Personally, I've gotten used to on screen keyboards and use a phone with a larger screen to have sufficient visual space. I need a large screen anyway so adding a hwkb would make it too cumbersome. Hackers keyboard gives you all they keys you most likely will need (tab, Ctrl..). I actually change keyboards depending on application. Hard to do with a hwkb! Regardless, adding a hwkb would be a fun project to try.

http://www.bhphotovideo.com/bnh/cont...C&Q=&A=details

Keyboard its little loud but take a look at Motorola Photon Q.

Yep, we have the Priv. This may be theoretically an option once it gets cheaper (it's a new device after all)

<edit> Priv will have a locked bootloader and extra systems to yield the device unrootable. And you can't chroot GNU/Linux without root access, AFAIK</edit>

A quick search yields Motorola Photon Q is unbuyable.

BB10 OS was officially declared dead (RIM said they won't develop it any further and stick to Android). So BB Passport seems a dead-end device.

How big is the impact of a BT keyboard on the battery life?

Well, on my note 4 the battery lasted about 1 or 2 days, before the i used the bt keyboard.
Now it's about the same. However i keep the BT keyboard off when i'm not using it.

And did you manage to bind some keyboard shortcuts, something like ctrl+s, ctrl+a?

Yes the shortcuts works by default