Forgive my ignorance, but what is so compelling about old PPC apps? I'm sure there are more modern alternatives out there.
Let me count the ways:
Software acquired by a competitor whose sole purpose was to kill it in favor of their software product: Aldus Freehand MX which was purchased by Adobe.
Software marketed by a company that does not like to put resources behind their Mac versions: Intuit's Quicken Deluxe, whose Mac version has been sorely ignored by the company in favor of their PC version. The last version of Quicken (not even Deluxe) was released in 2007 and was PowerPC. Even though Lion removed Rosetta capability in early 2011, it was not until March 2012 that Intuit finally released an Intel capable version of Quicken; and it was still the 2007 version, at that!
Software marketed to a segment of the marketplace that just does not have the resources to consistently update their software: the education segment of the software industry. Schools just cannot keep purchasing updates of educational software year after year. Students are given textbooks with CD-ROM educational tools that were written in the mid-2000s and are still PowerPC.
Orphaned software: Both Scrabble and Chessmaster 9000 have never been upgraded by their publishers and I think that their licenses to do so have expired.
Upgrades too expensive for the average user: Adobe's Creative Suite has become exceedingly expensive for the average user to upgrade.
... and so on!
So to summarize: there is examples where modern alternatives do not exist, or are too expensive for some users (especially students) to acquire.
