Absolutely not true.
There are a few single din systems with nav out there.
Kenwood has one that has a small single din sized display (possibly a 5000 something).
Thre are several flip out screens that are pretty nice.
Kenwood has one with a seven inch flip out that you can add nav to (possibly KVT 719 + G510 or something).
Pioneer makes a flip out seven inch (possibly the Z4)
Eclipse makes a flip ot with nav (AVN 7000)
And I found a little known product from:
Valor Multimedia (720w I think), that has a pretty compelling package for about $1000 (but cant find much info on company, I have a thread in the audio seciot about it).
A lot of installers will tell you that "you cant run the stock Bose speakers with an aftermarket head unit" which is blantly false. Its easier to just change out the Bose speakers if you are removing the Bose head unit and you will get much better sound with aftermarket speakers ($200), but, still, you can keep the Bose speakers and use an interface ($100) from the output on the new head to unit the stock speakers.
If you use a flip out screen, you may not like the way it blocks the ac vent if put into the space that the stock head unit comes out of. The solution is to move the heat and ac controls up to the old head unit spot and install the new head unit in the spot that the heat/ac controls were in. This is a simple move (the heat/ac controls are only a few inches deep, very easey to move).