There is a lot of evidence that quantum computers (the gate model) or, equivalently, quantum adiabatic computing is better than classical computing. All of it is based on a family of conjectures about the complexity classes P, BQP, and NP.
Scott Aaronson's blog is one of my go-to suggestions for rigorous introduction to the topic.
That's an important distinction because it's significantly more difficult to achieve a fundamental asymptotic improvement instead of an iterative speedup for the same factor. If a quantum computer completes a task with complexity O(2n) that a classical computer requires O(10n) to complete, you don't have quantum supremacy. If your quantum computer can accomplish a task in O(2n) that your classical computer needs O(2^n) to perform, you've got supremacy.
Given that nuance, I wouldn't say it may be a matter of time before we demonstrate quantum supremacy. There is still an undercurrent of skepticism in the research community.
Additionally, it’s worth keeping in mind that D-Wave machines aren’t true quantum computers in the sense that they can’t perform Grover’s or Shor’s algorithms.
