There's a difference between being able to withstand a bank run and being fully capitalised. To be fully capitalised the value of your assets needs to be greater than the value of your liabilities. You are allowed to value your assets at fair prices when you work this out. SVB did not have enough money by this measure and that is very very unusual for a bank. Despite this, you are right that even well capitalised banks cannot survive a bank. To survive a run you need to find a buyer for all of your assets right away, and you quickly run out of buyers, forcing the price down.