There are multiple examples in the literature of people leading perfectly ordinary lives whilst unknowingly having no more than 5% of the typical amount of brain matter (typically because of hydrocephalus). For example, https://www.science.org/doi/10.1126/science.7434023 from 1980.

The brain is indeed incredibly resilient - some kids with serious epilepsy get an entire hemisphere taken out - but which 5% you're left with matters enormously.