Just think about it this way.
If learning C++ and OpenGL are your goals:
- Set short term goals
- Make these goals easily attainable
- Understand it may take years to master or sometimes produce something worth selling
If making these games a reality is your goal:
- Grab Unity3D
- Go through tutorials
- Learn game pipelines and the basic C# unity requires
Learnings C++ and OpenGL is EXTREMELY valuable but also can take years before you are a competent C++/OpenGL programmer. Learning Unity3D can take months and can deploy on any platform. Learning Unity3D is also valuable, just not as transferable as raw C++/OpenGL.