The really short summary of it is that the web changed a lot in the last years (what a surprise…! 😉), and Drupal is not as cool as it was before. Solutions built using some kind of backend service and a JavaScript framework (known as the Jamstack) are the No1.
Fortunately, this is not unknown in the Drupal community. But according to Tim, the new role of Drupal is to be (only) the backend solution for a decoupled frontend. Because Drupal is perfect for that and (can be) better than the other (hosted) services/solutions.
“The backend has been solved, and is now fully commoditized. Our job today is to create amazing experiences that help our users navigate an increasingly noisy barrage of devices, networks, content, and tools.”
Tim recommends (to learn) to use React and/or Gatsby as the front-end solution(s). I know that they are the most famous tools for that, but let me advertise and recommend (the open-source!) Vue.js, Gridsome or Svelte as well (or instead of them)!
And I hope it is too early to bury Drupal theming so I recommend Compony.io too! (More on that later.)