Libraries or executables, it makes no difference. You are incentivized to use GPL because you wish to build on top of work that is GPL.

Obviously in large part that didn't happen, because of a cultural tendency to use more permissive licence variants (such as AGPL) for libraries, in the pragmatic hope that this would encourage their use even in proprietary programs, and therefore incentivize back-contributions from a wider audience. But this indeed halts the "virality" of the GPL, and so one is once again forced to conclude - incredibly - that Stallman was not radical enough...