However, you should not dismiss qualitative results out of hand.
If you do usability testing of the checkout flow with five participants and three actually verbalize the hypothesis during checkout (“hm, I’m not sure how to get to the next step here”, “I don’t see the button to continue”, after searching for 30s: “ah, there it is!” – after all of which a good moderater would also ask follow up questions to better understand why they think it was hard for them to find the way to the next step and what their expectations were) then that‘s plenty of evidence for the first part of the hypothesis, allowing you to move on to testing the second part. It would be madness to quantitatively verify the first part. A total waste of resources.
To be honest: with (hypothetical) evidence as clear as that from user research I would probably skip the A/B testing and go straight to implementing a solution if the problem is obvious enough and there are best practice examples. Only if designers are unsure about whether their proposed solution to the problem actually works would I consider testing that.
Also: quantitative studies are not the savior you want them to be. Especially if it’s about details in the perception of users … and that’s coming from me, a user researcher who loves to do quantitative product evaluation and isn’t even all that firm in all qualitative methods.