I think popularity based ranking should absolutely be optional and a toggle. I think it's reasonable to have default rank be popularity, but in my opinion, the value of a model like this is finding obscure tracks. I also would definitely like seeing an exposed toggle, and maybe automatically toggle it off when someone presses refresh?
Sure, a toggle is one option. The main point is to introduce popularity as a factor and ultimately the best UI and defaults are best decided through A-B testing.
I'd also add that it's usually a good idea to incorporate this setting into the URL, regardless of the UI, so that a specific order can be bookmarked and shared.
