I have two G4s on dual boot OSX, OS9.
I honestly prefer OS9.
Far more straight forwards, more customisable, etc.
I also use old Media100 hardware and software.
The difference is, I can have a professional video editing suite with component, svideo, composite or firewire inputs and outputs, and an external Monitor for video footage. I currently bought a 2nd hand Mac with all this for £250. A modern Media100 system for OSX would cost in the region of £3000-4000. ( About £2000 for a new Mac, and £2000 for Media100 HDe ). Even if I didn't want the new HD Kona Card, and just bought the tune up to upgrade my legacy M100i & P6000 to the OSX edition, it'd cost me £300 for just a new license key, when I can get a whole new editing suite for under £250 (another G4 kitted out with what I need).
So yes, I am successfully using two G4 Macs, with OS9 for professional purposes. The only difference is, I can dual boot anytime I want into OSX, and use Cleaner, InDesign, Photoshop, etc [ although I also have Photoshop, After Effects and Boris FX on OS9 also ] as both my G4s are dual boot machines, and can run both OSs.
So in a way I have the best of both OS systems. But I can certainly understand how OS9 can still be useful. For me, it's a cheap and cheerful option, and I find it much more stable than my Windows98SE machine, although I still use Windows for all my web browsing and website creation stuff.
Actually, I use OS9 for video post production, OSX for Print jobs, and Windows98 for web jobs (and also some print stuff).
OSX is still useful for stuff like Flip4Mac, but I do find myself trying to do things I can do easily in OS9, that I just can't in OSX, or at least haven't figured out how to yet. Like create a desktop shortcut for switching startup disk. I've got one in OS9, but the best I can do in OSX is a Systems Preferences shortcut, I still have to click on Startup Disk when it opens...
Also, on OS9 I have all my programs/apps in a pop-up folder, and also in a submenu of the main Apple icon, ala Win98's Startbar, but in OSX, I've just got a very, very crowded Dock, and it doesn't look as tidy. =P
Other than that, Safari is much better than OS9's iCab. ^_^. Heehee.
I still like both OSs though.
My point in writing was to say people can still use and love OS9. ^_^ It can still have its uses.
But it depends what you do - some things may now work perfectly well in OSX to justify an upgrade. That said, it's probably best to wait for UB versions of your professional apps and then get a Mac Pro if you're really gonna upgrade....
As for your boss...she's probably just biased. ^_^, fell in love with one system, and doesn't want to change.
P.S. Oh, also, I miss the little program switcher you get in the top right corner in OS9...now instead I have to look for little black arrows underneath icons on OSX, and it's just not as easy as the pop up menu in the top right of OS9 that lets you switch to any running program, hide or unhide, show all, etc. But I'm sure I'll adjust with time...