The project with closed source code can remain terrible forever even if there are programmers willing to improve it.

Here's an example: Looking for a WebExtention alternative to greasemonkey on Firefox. (I know a port is being worked on, but let's ignore that for now)

Your options are tampermonkey (closed source), and violentmonkey (open source)

Tampermonkey has been around longer, and probably has more features. Which do I choose?

I choose violentmonkey, because for all I know tampermonkey's code is garbage, and full of spyware. If violentmonkey doesn't meet my requirements, I can make it meet my requirements by coding the feature myself, or paying someone to add the feature.