My thoughts on abandonware:
1. If the abandoned software in question is from an indie developer who is still reachable by email, then just email them and ask how much they want for a license of the old version because you run it on legacy hardware. Most developers, even commercial software mega-forces are happy to keep you running the software on older hardware. They know that one day you WILL upgrade your Mac and will likely want to run the latest version of their app. So, it's just good business to offer unofficial support to the niche user. They may even give out a free serial, or ask just $5 or so for the license.
2. If the developer handed out a free serial, ask them if you can share it with other legacy users. If they give you permission, keep the email as proof that you're not breaking any laws. If the developer didn't give out a free serial, please don't share your license.
3. If the developer doesn't answer in a week or so, go grab a serial number from the usual sources and get on with it. No point losing sleep over it.
4. If the software is totally abandoned and the developer has disappeared off the face of the earth, it makes it impossible to pay them, and they don't expect to ever see any payment for their old software.. See point 3.
There are no royalty collection agencies for software development as there are for music, movies, photography, artwork and books. Unfortunately, if a developer dies, their family will not be receiving royalty checks for their work. This is a shame, however software is typically seen as a product and not a creative work which deserves royalties.
I would argue that software development is pure creativity on many levels, but that's not going to change the perception of the general user.
-AphoticD