A fruitfly study suggests that losing neurons is not necessarily a bad thing. The study used fruitflies genetically engineered to express human amyloid-beta proteins in their brains. When neuronal death was blocked, the flies developed even worse memory problems, worse motor coordination problems, died earlier and their brain degenerated faster. However, when the normal process of cell competition was enhanced, the flies showed an impressive recovery.
Cell competition is a cell quality control mechanism, by which fitter cells trigger the suicide of less fit ones. Research has shown that cell competition is a normal, powerful anti-aging mechanism.
The findings suggest that neuron loss reflects the brain protecting itself from defective neurons, not something that should be prevented. (What we want, of course, is for the neurons not to be damaged.)
(2018). Culling Less Fit Neurons Protects against Amyloid-β-Induced Brain Damage and Cognitive and Motor Decline.
Cell Reports. 25(13), 3661 - 3673.e3.