>But if you watch the video of the Amazon Key annoucement[2], they mention multiple other scenarios: "maybe let in your dog walker, a friend, or maybe to let in a team of cleaning ninjas"

For which solutions from all the major lock companies (and others) already exist. I can give people who need access to my house custom codes on my Schlage lock. It is an internet-connected thing--although it doesn't really need to be that. And I have found punching in my code is easier than dealing with the app on my phone.