> he's not saying that the GPL has a poor legal basis, just that the FSF definition of a derivitive work hoes beyond where it has a good legal basis.
Yes, sorry. In this context I was abusively using "GPL" instead of: "the GPL difference compared to the LGPL".
> the idea that software A could be considered derived from library B one day (because it only works with library B), and then the next day someone releases library C with the same interface as library B changing the status of Software A to no longer be considered a derivative work of library B (because it now works with both library B and library C) seems like nonsense to me.
Yes it would be nonsense.
What makes perfect sense however (whether it is legally correct or not) is that the status of software A in your example does not actually change. Because everything in your example is derived from library B, including library C since it is cloning B. I am not saying this is legally correct (I am not a lawyer). But it is simple, logical, consistent and makes sense.