Right, heightmap terrain would cover a lot of needs, but it's not the same as a voxel terrain, and another person might use their own meshes. For a completely indoor game, terrain might not be needed. But it's certainly not needed to have 3 different terrain systems bloating the engine.
One solution could be the terrain editor is built-in, but needs to be installed. That would not be much different from a plugin, though, it would just mean there's a built in node for it. I still think plugins are the best way.