Like someone else brought in a different comment thread, using sales as a metric for "best" would infer that McDonald's burgers are the best you can have, ever.
That's not quite right. The comment was really showing the the definition of "best" can vary based on a lot of factors. It is up to the reader's imagination to account for real life constraints such as time and budget in a burger purchase decision. If you have 7 minutes and the McD's across the street is the only burger place in town, they are the best choice as all the other burger shops result in having 0 burger.
The argument here is that they are not strictly related to quality.
Are you gonna tell me the PT Cruiser, random example, was a quality vehicle because of its astronomical sales for years?
Not understanding why is popularity used as an argument for good sales. It's not. Tesla cars being well made (or not) is not a reason why they sell.