Are there software licenses out there that guarantee to the user the ability to receive updates and improvements to a piece of open source software?
I don't know of any license other than the GPL that provides this guarantee to the user.
I know that I'm more motivated to learn a new tool when it is protected by the GPL because of this guarantee that future versions will also be open source.
When I license my own software, I choose the Affero GPLv3 because it provides this extra guarantee to the users, even if the software serving the user is on a webserver.