I suspect that OP knew the answer to the question before he even asked it, since the answer is quite obvious. The N9 is not a suitable replacement for the N900, except in those cases where the user does not require anything other than a typical smartphone. This also applies to the N950 IMO, since they run the same software, and that software is not designed to be used with a HWKB, which limits the advantage of having one. Also, I find the HWKB on the N950 to be cumbersome for handheld typing. It's too spacious for its own good, which means that your thumbs need to travel much further for each keypress. It does however make it easier to use on a table/desk. In all honesty, I prefer the keyboard on the N900.

N900 + more RAM + fast CPU = WIN!