You've just traded one problem for another. Now the wizard has to worry about someone stealing the focus and holding it for ransom.A higher level mage would probably keep his book in a Leomund's Check (or whatever they call that spell in 5e). Only available if you speak the password, which they won't get if he/shes dead.
You've just traded one problem for another. Now the wizard has to worry about someone stealing the focus and holding it for ransom.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.