Runtime linking doesn't always equals derivative work. Some GPL enthusiasts would like it to be that way but it doesn't mean it is. As far as I know it's a murky legal issue. I forget the exact case but one counter-example was: you have proprietary library A. Someone makes a GPL implementation B with a compatible interface. You ship software C with instructions that users can use either library A or B. An example of this would be BLAS libraries, which have both proprietary and open-source versions. Software C is obviously not a derivative work of GPL library B.

Now, the risk that a court might decide your software is a GPL derivative because it links GPL software might be enough to dissuade your company from using GPL software altogether.

LGPL makes it explicit that you can link against the software without making your software GPL/LGPL so it removes that risk.

And that's not what the AGPL is about. It's not extending the definition of what a derived work is, that is completely outside the hands of the license, it's a matter of copyright law. The GPL says if you distribute GPL (and by extension GPL-derived software) you must distribute the sources too. The AGPL says if a user accesses AGPL (and by extension AGPL-derived software) over the network, you must distribute the sources to that user. It doesn't mean that if a user uses unrelated software to access AGPL software, that unrelated software is somehow derived from the AGPL software.