>The scenario that worries me is that years from now, Google and Amazon may be coerced into turning over what I said yesterday

Amazon has said that they're only recording what happens after the hotword is triggered. That's why they can't do custom hotwords - listening for "hey alexa" is built right into the hardware and it doesn't do anything until it hears that. Unless they're flat-out lying, they can't provide any information about what you said yesterday to anybody because they don't have that data.