Let's say you were working on a plugin and spent I don't know how many thousands of man-hours developing it. Now let's say that the creator of ST was hit by a car. You would be fucked.
Fucked? You would keep using ST as it is until it would no longer run on the current version of the OS. The product wouldn't stop working the day the author quit working on it.
With an open source project, a sufficiently interested third-party has at least the legal right to pick up development of the code base that their work depends on (or to pay the developer of their choice to do so.)
With a closed source project, that's not the case.
