Just a small note that might help the rationality of things to keep in mind - this for GM4Him, since it seemed to be something on your mind - a spellcasting focus is relevant only for spells with material components, because it is what we use to replace those; if you don't want to keep track of how many tentacles you're keeping in your pocket, or how many handfuls of bat poop you have left, and you'd rather not think about that at all - that is precisely what a spellcasting focus is for; it specifically replaces the need for worrying about material components and acts as the 'stand in' for all of them, with exception of cost-listed components.
This also means that stealing your enemy's casting focus doesn't disable them other than blocking the specific spells that have material components which they don't otherwise have to hand - so even if you steal the wizard's staff, they can still cast time stop or prismatic wall, or power word kill, incendiary cloud, finger of death, prismatic spray, or any number of the dozens and dozens of other spells that don't have a material component - which it might surprise a lot of folks to know is actually over 40% of all spells currently in 5e (43.5% - 241 out of 554 have no material requirement; and that's not accounting for the ones whose 'material component' is 'a pinch of dirt' or 'a twig' or 'a bit of fluff'; the majority of ranger spells with material components have incredibly simple components like that, for example) ^.^
And besides, and self-respecting spellcaster always keeps backup casting focus in some deeply personal location that won't be found short of a cavity search... it's just pragmatic.
