While you don’t say what Mac OS version you are on, if there are drivers for the M-Audio interfaces available on their website for your system version, it should work. I have a Midiman 2 port midi interface that still has drivers available. for 10.8.5.
You’ll be able to use the built-in sound if you don’t try to tax it too much- I don’t know exactly how much since I use an external device (which is 6 years old).
If the Promix still works, you could conceivably connect it to Cubase as a Generic Remote via a midi interface, so it could possibly be useful, maybe, if you’re the type who enjoys configuring the Generic Remote…
I like to use my gear as long as possible and try not to get sucked into buying stuff just because its shiny.
+1
For at least 10 years audio quality of (semi-)pro digital audio gear has been so good that in 99% of cases the weakest link is somewhere else (usually somewhere between the keyboard and the chair). Those “old” M-adio interfaces are completely adequate for making great music.