It seems like if you discover a need, then making a publicly available plugin to fill that need could be a good idea.
Compared to other ways of generating a profit, such as building a membership website, or one supported by affiliate advertising, creating plugins for profit should probably be low on your list.
The reasons:
- You’ll have to maintain updates well into the future as the world of WordPress continues to evolve, new ways to inject malware are invented, and needs and technologies change.
- Most web designers love free plugins. Few will spend money for ‘pro’ versions or plugin subscriptions.
- The most important reason is technical support. Until you’ve made a plugin for the public, you may not believe how much technical support will be needed for even the simplest plugin. People will demand that you tell them information that is already in your documentation. People will try to use your plugin for purposes other than for what it was intended, and there’ll be no end to extenuating circumstances having to do with everything from unexpected theme color combinations to concurrent resource usage.
You may want to just give your plugin away. That’s very nice of you. However, much of the same information applies. Even though it’s free, you’ll probably feel an obligation to keep it up to date, and help people with tech support.