How is it that we can lose so many neurons without massive memory effects?

Answer 1

While it may be true that we lose many neurons (especially as we age), and it is true that neurons don't divide like other cells, it is also true that new neurons can and are being made via neurogenesis. It is also true, that axonal branching takes place so that in many circumstances, the existing neurons innervate additional targets. All in all, in the absence of pathogenesis (dementia, etc.), the memory loss isn't as great as might be predicted, although some loss is inevitable with aging.

Answer 2

That's where the "we only use 10% of our brain" crowd goes wrong. My understanding is that we have and need massive redundancy and rewiring in our brain to cover for damage.