In my opinion Linux will never replace the Unix in corporate market. This statement a bit different with statement that come from university student or of course linux activist. Here's my reason:

1. Linux doesnt have a real server machine.
Comparing to HP-UX with the HP 9000 series or IBM AIXwith IBM System p5 servers or even Sun Solaris 10 with Solaris 25K machine.

2. There is no All in One system.
Exchange maybe best in email or messaging apps, but for high and critical business transaction maybe IBM OS400 its far more better. But linux claim that it has a very good desktop app, has a very strong web server, ability for perform critical business. Let me ask you, and answer it honestly, is that any all in one system?

3. Linux company didnt have a stable financial, network and support for the company, compare to others mature players

Tobe continued, and the list will getting longer