Adam,
It may not be helpful. but I have been running FCS3 and all it's apps (including STP) on OSX 10.6.8 with no glitches of any kind, it is not the apps as Studio X points out. I too don't like to spend time wiping everything down & reinstalling.
Fortunately, Macs have always played nicely with me (I have owned both PC's & Macs since the beginning - after the abacus & slide rule went out of style). PC's are of course, more ornery and get corrupted more often, at least in my experience, which is why my current PC plays second fiddle to my Mac Pro & MacBookPro.
At any rate, as GT notes, the original question about a replacemnt for STP is still open to a degree in this thread. The cold hard fact is that APPLE does not have at this time, any "replacement" for STP, and it's commonly accepted by the experts that there is no intention in Apple to work on one.
It is worth noting that STP can export in a variety of formats (Aiff, wav. MP3, Dolby ACC, Next, Sound Designer and even compressor). If you choose not to reinstall as Studio X suggested, you could pick any of a number of software platforms which you'll have to purchase, and then export the work already done in STP to those.
Yes, it's a P.I.T.A., but Apple, like all Kings of the Jungle, does as it wants, when it wants, without regard for what YOU or I want. Join the "wish hadn't done that " club, have 2 or 3 adult beverages of your choice and adapt as best you can.
Ken