Can we build mid level Whole game by blueprints?
It is impossible to say unequivocally. Blueprints can do almost everything that is INSIDE the engine. For example, move characters, detect collisions, spawn new actors, save game in built-in way and so on. But if you need doing something OUTSIDE the engine - open the file, work with the database, make some non-standard network interaction - here you must either download special plugins that can be broken by the next update or use C++.
I made a game using only blueprint and it’s selling on Steam, so totally possible.
You have plugins to do things outside the engine, or you can do a little C++ for that.
The only time you really need C++ badly is for efficiency, which only becomes an issue if you’re really pushing some aspect of the engine. For instance making a very large voxel terrain.