Yeah, GPL is much better, but it's still no guarantee that they did actually foster a community equipped to continue development.

They can (knowingly) fuck that up by:

• Not pulishing documentation.
• Not accepting outside contributions.
• Accepting outside contributions, but only under a Contributor License Agreement (CLA) which allows them to re-license the contributed code.
• Not mentoring outside contributors.
• Not open-sourcing everything that's required to actually make the software useful.
• Making it only profitable for themselves (e.g. Android pays for itself via the Google ads that you can integrate into your app with a handful of clicks)
• Putting more development effort in than any fork could.

And these strategies work especially well, if you're developing:

• a platform (where a competing fork will need to stay compatible with you at first)
• a centralized communication service (where you have natural friction, because no one wants to leave, because none of their contacts are elsewhere)
• security-critical software (where using a fork is potentially risky)