Essentially every encryption scheme in history has been broken. It's a bit silly to think that "well designed modern" makes it safe. The others were all well designed and modern at one time, too. The only scheme that's truly unbreakable is a one-time pad and that's... well... not really practically useable.

Personally, that's not a risk that I would care about (especially in the context of an Echo Dot or even an iPhone), but deleting the keys will not be better than inaccessible forever.