I'll try to tackle this question from a developmental vantage point.
You hit the nail on the head by saying
...the brain is
just neural connections...
Indeed, the power of the brain is based on the connectivity of its neurons. The number of connections is what counts, not just absolute neuronal numbers. In fact, more than 50% of neurons are lost during development, as a result of limiting trophic support from the target tissue they are destined to innervate. This neuronal cell death facilitates proper connections in the nervous system. The apoptotic cells are indeed unwanted in the nervous system, and apoptosis continues throughout life and is the central mechanism for the removal of surplus, unwanted, damaged or aged cells. (Mazarakis, 1998). Leaving them could result in aberrant functionality. For instance, it is claimed that synesthesia may be linked to incomplete neural cell death in development, leading to aberrant connections between the senses. Many sysnesthetes for instance see certain numbers appear in specific colors. These cross-sensory associations are thought to be caused by faulty cell death between the respective sensory systems in the central nervous system (the brain).
In effect, adding neurons to an existing functioning brain is expected to lead to disruptive brain function, rather than improvement.
The practical limitations of injecting additional neurons are mountainous -
- Where would you obtain viable neurons from? Stem cells?
- In the case of stem cells there's a chance of uncontrolled growth (cancer);
- If the neurons don't come from a genetic identical individual there's the issue of immunological graft rejection;
- How would you deliver the neurons to their appropriate position in the brain? Just drill a diffuse set of holes in the skull? How would you reach the deeper layers of the brain? How would you contain the neurons to the brain and prevent 'leakage' to unwanted sites in the body, such as the blood stream?
- Mazarakis, Archives Disease Childhood (1997); 77:F165–F70