The thing is, if one ore more major exchanges go down, there might be a panic, which rapidly decreases the selling price and thus, the value of BTC. So where is your store or value, if you can’t withdraw?

What if a law is passed that allows BTC currency conversion only to a small amount per day, because of whatever reason?

Currencies like the USD don’t have this property. There is no need to exchange your money to make it valuable.