More importantly for Linux, curl, etc., almost nobody uses Linux from Linus or curl from Daniel. You get it from your distributor, and in the case of Linux it usually comes with quite a few patches. These distributors (even the all-volunteer ones like Debian) are projects involving lots of people and clearly defined procedures for what to do if one of their maintainers stops being able to contribute.
A good example is glibc; several years back, a huge number of people were using the eglibc fork, not because glibc upstream (Ulrich Drepper) stopped being able to do releases, but simply because he was refusing patches for architectures he didn't like and other similar changes. Very few end users even noticed that they weren't using "real" glibc. (Ulrich has now stepped down and the eglibc changes have been merged back in.)
A good example is glibc; several years back, a huge number of people were using the eglibc fork, not because glibc upstream (Ulrich Drepper) stopped being able to do releases, but simply because he was refusing patches for architectures he didn't like and other similar changes. Very few end users even noticed that they weren't using "real" glibc. (Ulrich has now stepped down and the eglibc changes have been merged back in.)