Naturally this is a risk for anyone with any assets.
Extortion is a risk for anyone with assets, sure.
However with an asset that is pseudo anonymous, with no knowledge of who posesses the asset (private key), Bitcoin creates a totally unprescedented opportunity for extortion.
I cannot envisage any other situation where you could steal $400m from one individual and nobody raise an eyebrow or even know it ever occured, can you?
It's 'pseudonymous' not 'pseudo anonymous'.
Kidnaping or extorting in any other way is illegal, so claiming you could just extort Satoshi with zero consequences is like claiming you could extort physical cash from someone and get away with it. Sure, it's possible, but to pull that off, you would still need to make sure you leave no witnesses/evidence + good luck explaining to authorities how the hell have you become multimillionaire overnight (when you decide to sell those BTC).
It's probably easier for extortionist to target any other, wealthy person and demand more 'reasonable' amount of BTC as ransom.